1. How to Sideboard
Over the course of a tournament, you will usually play more games WITH a sideboard than without one. Since a match is determined by the first person to win two games, when both players aren't playing exceptionally long games, every game must go to at least two games, and often a third game. Your second games and third games are played with a sideboard.
Your sideboard isn't simply 15 extra cards that you have in your collection, or cards that you wanted to play in your maindeck 60, but couldn't find room. It's there so that after you play your first game, you can alter your deck slightly to improve your matchup for game two. This usually means that you will include cards that are very narrow - good against some decks, poor against others (if they were good against everything, they probably should be in your main deck).
After you actually pick cards, you need to decide ahead of time (not during a match when you only have a few moments to make changes to your deck) what will go in and what will go out. Usually, you only want to bring in a few cards and take out a few cards; doing more than that dilutes your deck and you may find too many hands filled with answers without a way to win as intended.
2. How to Build a Deck on a Budget
There are generally two methods to creating a deck on a budget: you can take a top-down or bottom-up design process. In the former, you are taking a pre-existing deck that has done well, but includes many expensive cards, and try to find budget substitutes. In the latter, you take budget cards and combine them together to form a cohesive deck.
Top-down deck design:
In a top-down design, we take a pre-existing (non-budget deck) and try to make it fit our needs by removing and replacing more expensive cards with cheaper versions. The most important thing to keep in mind is that there may not always be a good substitute, so often you aren't looking to find the next closest copy to a card, but rather the next best card for the deck that goes well with the rest of the cards. If you can't afford a planeswalker that produces tokens, playing a sorcery that puts two 1/1's into play probably isn't a good replacement; you're better off playing a midrange creature that will keep pressure on your opponent and deal more damage than a few token creatures.
Bottom-up deck design:
In a bottom-up design, we try to form a cohesive deck by gathering a bunch of affordable cards that work well together, until we have a good list of sixty cards. You make sure you have a good number of creatures and spells (particularly removal), and then add in some land to fill out the deck, paying special attention to the costs of all the cards so that your land will do their best to ensure you can play everything.
3. Decks Under $20
4. Decks Under $50
5. Decks Under $100
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