Depending on how much time you have, you can do any or all of these things:

• Make some notes after the game as soon as you can 

  1. What were you thinking?  First of all, try and write down the main things you thought about during the game.  Who was better or worse at each stage?  Which moves were you sure of, which were you unsure of?  Which moves were good or not so good?  What were the alternative moves you thought about?  This records your own thoughts, and you can come back and do the rest later. Can you tell the story of the game in a couple of sentences?
  2. What does the textbook say?  Which side turned off the main line of the opening first? You might want to look up what you should have played in the opening.  You don’t have to know every move, but it’s a good goal always to have your opponent be the one who runs out of book moves first.
  3. What went wrong? Next, try and find improvements — of the alternative moves you thought about, do you now think any of them were better than what you played?  Can you find a better move than any of the ones you thought about at the time? Focus on (a) big mistakes not little ones, and (b) positions where the evaluation changed significantly (winning → equal, equal → losing). These moments where the expected result changes are the ones to learn from.
  4. Better next time? If you find some improvements in your later play, try and remember the point for later games.  Improvement comes from fixing thinking errors, not just book moves or noticing a tactic.

• When you have a group of reviewed games:

  1. Classify your mistakes by type
     For each serious error, label it e.g. Tactical mistake, Positional misunderstanding, Wrong or absent plan, Time trouble / too fast, Opening knowledge,Endgame technique.  Are there themes?  Patterns matter more than individual blunders.  And a hard question: why did you make that mistake?  If you can answer that question, you can think about a cure.

  2. Turn ‘better next time’ lessons into rules or reminders
     e.g. “Don’t push flank pawns when behind in development”,  “In endgames, activate the king earlier — no need to castle”

  3. Track recurring themes across games
     Keep a simple list of common issues (e.g. “missed back-rank tactics,” “overoptimistic attacks”). Again, then you can think about a cure.

• Compare with model games in similar positions

  If a game featured a common structure or opening, look at how strong players handle those positions. This helps you replace any mistakes or confusion with better ideas.

And most importantly:

• You can do all this on paper or using a computer. 

BUT

Only turn on a chess engine to check your conclusions
Use the engine to:

• 1. Confirm or refute your analysis

  2. Reveal tactical shots you missed

  3. Suggest better plans
     Don’t just click through moves—pause and understand why the engine prefers something

  4. If you are doing the thinking, your thinking can improve.  If the computer is doing all the work, then I can expect only the computer to improve!