Almost every serious aspirant is told to maintain an error log. Most do, for about three weeks, and then it becomes a list of question numbers that nobody ever reads again. The format is what decides whether it works.
Record the cause, not the question
Writing down that you got question 34 wrong is close to useless. Writing down why is the entire value. Use four fixed categories: did not know the concept, knew it but misread the question, knew it but made a calculation error, or ran out of time. Each demands a different response, and lumping them together hides that.
Review weekly, not eventually
Set a fixed slot — Sunday evening works for most students — and re-solve every question from the week's log without looking at the solution. Anything you get wrong a second time moves to a separate short list. That short list is the single most valuable page you own in the final month.
Watch for the pattern
After a month, count the categories. A log dominated by calculation errors points to practice under time pressure, not more theory. A log dominated by misreading points to a comprehension habit that can be fixed in a fortnight. Students frequently discover their real problem is not the one they assumed.
Keep it short
One line per error. If maintaining the log takes longer than solving the questions, it will be abandoned — and an abandoned log helps nobody.