This is the right answer. If you think you understand something, pretend that you have to teach it to a beginner and write a short lecture/presentation to give

This is the right answer. If you think you understand something, pretend that you have to teach it to a beginner and write a short lecture/presentation to give them. Explain what they'll be doing, why, and why it is important. Do it in which ever language is most comfortable. If you can prepare that lesson then great. If you cannot, then your initial assumption that you understood the material was incorrect. Mathematicians write material like this all the time (they like to call it "expository material").