This is the error log
I created. I find it useful because I added a timer, so I am able to time myself on each question. You simply click start, and then end when you are done answering. Then you can type in the time under "Question Time". I included 2 examples of questions I had attempted.
You can personally assign your own difficulty rating as well to help you keep track of what you find the hardest.
Also note the hyperlinks to gmat club questions which I think could be very useful in quickly finding gmat club posts that may be of some interest.
I know it's an "error" log. I think it is more useful as a question log. Log every single question you do. You may have guessed right on a question, or werent 100% sure, so you can take notes on each and every question. If you got a question right but you took too long, that should also throw up a red flag.
Make sure you enable macros so the timer works !!