1) Have a strategy!
This is the most important point of building a deck. The questions you should ask yourself before building a deck is: How do I want to score victory in a match and how do I plan to stay alive until this happens? Some decks play a self destruction strategy and see their life only as an additional resource (burn or suicide for example) others try to control every aspect of their opponents game. Keep those informations in your mind for the whole deck-building process. Always remember that you will have an opponent that also wants to win the game and doesn't simply sit there and whines "Please go on and defeat me"
There are some basic strategies to achieve a victor and lots of variants in those:
• Aggro: You try to reduce your opponent’s life from 20 to 0 as quickly as possible, rather than emphasize a long-term plan. The strength of this strategy is to put pressure on your opponent, outmanoeuvre him and force him to react to your actions rather them let him play his own game. Typical colours for this strategy are white, red or green.
• Control: You try to avoid racing while attempting to slow down the game and control what your opponent does. The strength of this strategy is your ability to devalue your opponent’s cards. As the game progresses, you will be able to take advantage of slower, more powerful, cards. Typical colours for this strategy would be blue or black.
• Combo: You try to utilize the interaction of two or more cards (a "combination") to create a powerful effect that either wins the game immediately or creates a situation that leads to a win. The deck should be reliable enough to produce the combo on a regular basis, the combo should be strong enough to win a game and the deck should be able to use the combo fast enough to win before the opponent.
• Midrange: You try to hinder your opponent’s game and start to build up pressure. As the game progresses, you will be able to take advantage of slower, more powerful, cards. This is a mixture between Aggro and Control. Against aggressive decks you will want to play more control-ish while against control decks you will want to play more aggressive.
• Mill: You try to reduce your opponent’s library to 0 and then forcing him to draw a card. The strength of this strategy is that you will put many cards your opponent wants to draw into his graveyard from where their hard to reach for him thus disturbing his plan. The typical colour for this strategy is blue.
• Special strategy’s: There are some cards that offer you alternative "Win Con" (winning conditions): "When XYZ happens, you win the game"
2) Play efficient cards!
This sounds easier than it is and has a lot to do with experience. The choice of cards depends on the strategy you have chosen. Ask yourself some questions before you use a card in your deck:
• Does this card fit into my planed strategy?
• Does this card have any synergies with the cards I’m already using, making them stronger?
• Is there a card which has the same effect but is less expensive to play or more flexible?
• Are there better options for this card in another colour?
Especially the last question is a tough one. There might be many cards in other colours, that might be better that cards you're already using. But adding another colour to your deck might also make it more difficult to get the right amount and combination of mana to play all those spells.
Also try to evaluate if a card will still be useful if drawn in later turns. You don’t want a card that lets your opponent discard X cards, when he only has one card in left in his hand, which could be removed with a much cheaper spell.
As rule of thumb you should also consider that aggressive decks rarely use cards with mana-costs higher than 4 or 5. Even control or midrange builds use few of those cards, because they are expensive and if you play them you normally want to have some mana left, so that you're still able to react on an action your opponent makes. Last, cards that have an impact on the game the moment they come into play are to be favoured over such ones that don't do so.
Some examples for mistakes:
•„I Play that card because I like the picture“
• “I’m using a spell that prevents all combat damage (like “Fog”) and something that gives me life once (like “Sacred Nectar”)”… Such spells won’t help you because you don't solve the problem you're having but just push it one turn further away. It’s better to handle the problem.
• “I play an aggressive deck with lots of small creatures, but in the case the games takes longer, I use some really big, expensive, ones”…If you want to win the game as fast as possible you don’t need the big creatures. By the time you will be able to cast those, you should already have won the game.
• “I play an aggressive deck which is supposed to overrun my opponent fast. But to defend myself against enemy creatures, I play some defenders and spells which give me life”… If you want to overrun your enemy, you should force him to play defensive, so he has to block with his creatures. To achieve this, it’s better to play aggressive spells and creatures, it’s regardless if you have 15 or 1 life at the time win the game.
• “I’m playing this big 8/8 beater for 8 mana to finish my opponent”… Those big, expensive, creatures look impressive when they get on the battlefield. But if they don’t have any impact on the board the moment they enter, or you aren't able to protect them for a certain amount of turns, they will in most cases be dead before they can do something.
• “I want to defend from the beginning and play my big creatures late. To defend myself against flying creatures I play spell A, against small creatures spell B, against artifacts spell C (and so on)”…Don't use to specialised spells that highly depend on the situation and cannot be used in other ways. There’s a good chance that your opponent simply won’t play e.g. any flying creatures or artifacts, which will leave you with a card in your hand that you cannot use through the entire game.
• “I play a combo-deck is based on 4 different cards . First I need creature A and B, the Artifact C and the, if I play spell D I can draw as many cards as I want”…Combos that use 3 or more cards are rather fragile because the chance to have them in your hand when you need them is very small. Combos with creatures are fragile, because creatures can easily be removed. Also a combo should end the game directly and not just let you draw cards, or give you life.
• “I’m playing mill and use this incredible, but expensive, card that let’s me put half my opponent’s library into his graveyard ”... The card [[Traumatize]] is a good one to explain this failure. It costs 5 mana and removes half your opponents library. If you decided to play mill strategy, you will have removed a great deal of your opponents cards from his library until you will be able to play this card. If you would be able to cast this spell on your first turn, then it would be very mighty, removing 26 cards from your opponents library. But if you cast this on turn 6,7,8 (Remember you don't draw a land on each of your turns, so getting the mana for this will take some time, see "mana-curve") then you will have already have removed a great deal of cards from your opponents library, making [[Traumatize]] less and less effective. The card [[Fugue]] would be another example. Costing 5 mana to play, it allows you to let your opponent discard 3 cards from his hand. By the time you could play this there's a high chance that you opponent will have less than 3 cards in his hand, making it inefficient.
3) Have a mana- curve!
With the most decks you will be able to play a land in the first 3-4 turns. After that you will have an interval of 1-2 turns until you can play the next one. That means that you can play cards for 1-3 mana in your first turns but the expensive ones will come much later. Choose your cards so that you are able to play a card every turn. But be aware that too many cheap cards won’t help and too many expensive cards will slow your start, you need a good mixture.
4) Play cards 4 times!
You have a strategy and you have some cards that you want to draw to win this game, those cards are key-elements to your plan. So if your deck and strategy is built around certain key-cards you can question yourself “how often do I want to have those in my hand or on the battlefield?”. The answer in most of the cases should be “as often as possible”, so have as many of those in your deck as possible: 4 times. If it is part of your strategy to have e.g. a counterspell on your hand in turn 2, you should not just play one type of this spells 4 times but two or even more.
Cards that you don’t want to have on your opening hand, can be reduced to 2-3 copies. This ensures that you have a chance of drawing them later on when you need them. The same goes for legendary cards, because normally you don’t want to have more than one on the battlefield and perhaps a second one on your hand. If you have a high number of spells that let you draw cards, or you're using cards that let you search for other cards (so called "tutors"), you can further reduce those numbers. Another drawback of using a lot of cards only once or twice is that it is pretty hard to judge a deck that consists of an extreme amount of different cards. Most times one of the first comments you will hear (at least from me), if you don’t follow this rule or explain why you don’t follow it, is that you should play important cards 4 times.
5) A deck consists of approximate 20-22 lands!
This number can vary from deck to deck, but should never be much lower. It’s annoying to draw land after land in longer games, but if you don’t draw any on the start, the game will be over pretty fast. A deck that plays mana- ramp or that uses many cheap spells can get along with 18-20 lands. Decks with many expensive cards and the possibility to draw many extra cards can contain 22-24 lands
Each deck that consists of 2 or more colours should play some cards that guarantee that you have the right mana in the right moment. That counts particularly for aggressive decks which reliably need a certain amount of mana in a certain colour-combination very early. It’s probably the easiest to achieve that if you change from some standard lands to dual lands. Many of those have some drawback, like “comes into play tapped” or “pay 2 life or comes into play tapped” but usually it’s better to live with that, than not to be able to do anything. The other way is using spells that can search you a land or artifacts that fix this mana problem. But both of those need space in your deck and reduce the maximum number of cards available for your strategy.
Also lands that enable you to search your library for another land can be used in mono-colored decks to decrease the chance of drawing lands on later turns. If you play and use such a land (like “Evolving Wilds”) it will remove 1 additional land-card from your library that you can’t draw later on.
An expert article on this from WizKids:
http://magic.wizards.com/en/articles/archive/level-one/building-mana-base-2014-11-24
6) A deck consists of 60 cards!
You have a deck-built full of cards and now comes the hard part: Bringing it down to 60 cards. If you really can’t decide which card to remove, or you need an additional land to fix your mana- problem, it can be 61 or 62 but not more. At last you have a strategy and the more cards you have, the lower the chance is on drawing cards you want to have. After playing some games you will be able to identify cards that don't fit, and remove them, replace them, or reduce their number.
7) Write down those information's!
Each decks should include a deck- and how to play description. The deck description should involve things as the basic idea why you did build this deck, if there's any story behind it, budget, certain cardchoices, especially if they deviate from common lists, legality, or any other information you can give others that will look at your deck. Best do this while you are building the deck because after some days important information's simply may not be remembered.
The how to play description is crucial for beginners. An experienced or expert builder can take a look at your deck and see how it should work, but there also are a lot of beginners out there that maybe want to take a look at this and compare it with their own ideas/ decks. Even for more or less standard-decks there should be a description just because of this.
By using deck-tags you can avoid to put unnecessary information into the deck name and thus making it too long. The tags also allows you to filter the decks you have made for a certain type, very useful if the number of decks you've build here steadily grows and grows.
8) Sideboarding
A deck can never work effective against each and every opponent there is out there. This is where the sideboard come into play. Sideboarding helps you address the weaknesses of your deck against certain opponents. So cards which have effects that you don’t need every match belong into your sideboard. If this effect is additionally on a card it’s fine, otherwise it’s a waste of free slots in your strategy.
Good examples are cards that have/ give reach/ flying, prevent lifegain, destroy artifacts or enchantments and so on. If your opponent doesn’t use e.g. flyers, your 1/1 with reach is a 1/1 without any (useful) ability and thus unnecessary.
If you have lost a round try to analyse why and think if any cards in your sideboard might help to do something against that and which cards in your deck won't help you. Your opponent can, and most of the times will, do this too, to counter what he thinks you will be doing.
An expert article on this from WizKids:
http://magic.wizards.com/en/articles/archive/level-one/sideboard-2015-08-10
9) Test your deck!
After you have built your deck do some test-games, preferably against more than one deck and more than one opponent. You will see if your strategy works or if some cards are too expensive or ineffective. Change those, but don’t forget to stick with your strategy. You could also write down the results in your deck-description, so that other players will see how this deck behaves, and can address weak spots faster.