Pick one position from the game, a position where you was in doubt about what to do, that you're unsure of afterwards. First, try to write down what your thoughts about it were during the game, why you played what you played. Put those notes away.
Next day, set it up on a board. First think about for ~10 minutes, without moving the pieces. If you think of a concrete line (a reason why some move won't work, threats, possible ideas of what to do), write it down. Also look for candidate moves, write all of them down. Try to put in words what is going on in the position -- what's each side trying to do? Is that necessarily the only thing they could do, or are there other things they could try as well?
After the 10 minutes, start moving the pieces. Check your written down lines. Look for new ideas! After 15 more minutes you should have a good idea of what's going on, what works, what doesn't, and whether the move you played was good or not.
Take 5 minutes to check the most tactical lines on a computer, there may have been hidden tricks.
Now take your notes and compare.
Repeat after your next serious game.
If you get good at this, you can start doing it for two positions per game. Then three. There will also be positions that don't need half an hour. Eventually you'll be able to go through a whole game.
I found that I only got better at this after working through a lot of Yusupov positions -- first I had to learn how to look for the best move in a position I had on a board at home, then I could apply it to positions from my own games.
I find it impossible to review my own games. Even though stronger players strongly suggest to do this.
If I review a game, I would play the same move again, even if it was a losing move.
How do I do this?