How to build a chess opening repertoire from your own games
Most opening advice starts with a book or a course: pick a defence, memorise five hundred lines. Then you sit down to play and your opponents leave those lines on move six. A repertoire that starts from the games you already play is smaller, easier to remember and fixes the positions you really reach.
1. Collect the games you really play
Download your recent games; a few hundred are plenty. Blitz and rapid count: they show what you play when there is no time to think, which is what you actually know. Chess.com and Lichess both publish every player's games, so all you need is your username.
2. Turn them into a tree
Lay the games on top of each other, move by move, once for White and once for Black. The positions you reach again and again become thick branches; everything else is a twig. You will usually see that a large share of your games pass through a small number of positions. Those positions are your real repertoire, whether you chose it or not.
3. Choose one main move where you are to move
For every position you reach often with yourself to move, decide what your move is. Check it against an engine or an opening database, but you are not hunting for the engine's first line: you want a sound move whose idea you understand. If you have been switching between two moves, keep one as the main move and, if the other is good enough, keep it as an accepted alternative.
4. Prune
Cut what you do not want to play again: the gambit that keeps losing, the sideline you tried once at two in the morning. A repertoire is defined as much by what you removed as by what you kept, and a smaller tree is one you can remember.
5. Grow it one game at a time
After each game, find the first move where you left the tree. There are only two possibilities. Either it was a slip, and you review the move you meant to play; or it was a position your tree did not cover yet, and you choose a move and add it. Five minutes per game is enough, and the tree grows exactly where your opponents take you.
How deep should a repertoire go?
As deep as your games go. If your opponents leave theory on move seven, knowing move fifteen of the main line is wasted effort. Put the depth where the frequency is: ten well-understood moves in the position you meet every week are worth more than twenty in one you meet twice a year.
Doing it with ChessBonsai
ChessBonsai does these five steps for you. You type your Chess.com or Lichess username, it downloads your games, grows the tree, marks the move where each game left your line and lets you cut that move or accept it as an alternative. Then you train only your own branches.