You can find Error Log samples here on BTG, but you can also create your own. I'd make an Excel spreadsheet with four columns, to stand for four categories: Question, Concept Tested, Correct Answer and Why, and Reason for Mistake. For the "Question" tab, you may want to add just the location of the question, or you may want to type/copy and paste the entire question in the field.

Create tabs separated by question type. This will make reviewing so much easier, and will allow you to focus on only one question type at a time. For GMAT, the tabs should read: Problem Solving, Data Sufficiency, Sentence Correction, Reading Comp, and Critical Reasoning.

Add to the error log immediately after each study session. The Error Log will only work if you continue to add to it on a slow and steady basis. If you wait until you've finished half the study guide, there is no way you will want to type all of that information into your spreadsheet! Adding to it right after you finish the questions will help you better retain the correct process to solve.

Good luck!