Mainly practice. Once you've got a sense of your play style you can find openings that fit it and study the books on them, and you should do the endgame drills at some point (at least you should know how to win vs a lone king with a king and rook (easy), king and two bishops (tricky), king and pawn (position-dependent, not always possible) - I've literally never needed to do king, bishop and knight). But the main thing is to just play a lot of games.