Also have a good understanding of the more complex mechanics such as chaining, targeting, missing the timing, and different conjunctions.
Have a good balance of monsters, spells, and traps. A good ratio is 15-20 monsters, 10-15 spells, and 5-10 traps, but you don’t have to follow this exactly. Use effect monsters unless your deck focuses on normal monsters, since effect monsters are generally more useful. Get ones with good effects and stats, and don’t use too many high-level ones or ones you can’t summon easily. Have spells that destroy monsters, help you summon or search stuff, destroy spells and traps, and protect your monsters. Use traps that prevent summons, destroy stuff, negate stuff, and prevent attacks. Keep your deck at 40 cards, or as close to 40 as possible. Keeping you deck close to the lower limit will help you draw the cards you need to win. If decking out is a serious problem for your deck, you could increase it to 41 or 42.
Xyz monsters are the most summonable cards in the Extra Deck. Since they only require two monsters of the same level, nearly all decks can bring them out. If your deck has three or more easily summoned monsters of the same level, you should include at least a few Xyz monsters of that Rank. Synchro monsters will require a Tuner. Add the levels of your tuner(s) to your most commonly summoned monsters, and include some Synchro monsters of that level. Make sure you can fulfill any specific summoning requirements of the Synchro monsters, and that their effects will help your deck. Fusion monsters are more specialized. Only use them if your deck is designed around Fusion. Use the ones with good effects and materials that are not overly specific.
Spells and Traps that make you discard cards are usually very powerful, but could be bad in a deck that can’t maintain a large hand. On the other hand, paying life points or giving them to your opponent seems like a bad deal, but paying some LP is better than losing, and a few powerful attacks can easily take back any LP they’ve gained.
