Do The Math
There is a lot of different math that might matter to a Magic: The Gathering deck. For example, the most obvious is land count. You always want your deck to contain enough land cards that you can play them as often as you need them, but not so many that you’re drawing lands instead of cards you need to win the game. A good rule of thumb is to shoot for 2-in-5. This means that for every 5 cards in your deck, 2 of them should be lands. In a regular old 60 card deck, this means 24. In a 40 card deck, 17. In a 100 card deck, 40. Then, factor in how often you actually need those lands. How many lands do you need on the board to win? Only 3? Then you can drop on down from 24 to ~20. Do you need 10 lands? Yeesh, maybe run a few extra. What if your deck doesn’t actually need it’s lands for mana though? Do you run rocks? Dorks? See what you can count as a land and what you can’t.
Take it for a Spin
Maybe you aren’t confident in your math. Maybe you just want to get a preview of how your deck rolls. Whatever the reason, it’s always good to give your deck a test run. Once you put it together, draw a few opening hands. How do the land counts look? What about playing a few turns. Do you have no defense for 4 turns? You might die to aggro. See what your deck does when there’s nobody playing against it, and guarantee your deck will never lose to itself. That way, when you take it to an event, you’re only playing against your opponent, and not past you.