The Feynman technique, step by step

Pick one thing, not a whole chapter. "Why enzymes lower activation energy" is a topic. "Enzymes" is a syllabus. The narrower the topic, the more obvious your gaps become.

Write the explanation out in full sentences, in plain language, with the book closed. Use an example. Say why it matters. When you catch yourself reaching for a textbook phrase you cannot unpack, you have found a gap rather than a memory.

Speaking it out loud works too, but writing is harder to fake: a vague sentence looks vague on the page.

Reread just the part you could not explain, then close the book and finish that sentence in your own words. This is the step people skip. Rereading the whole chapter feels safer and teaches you almost nothing new.

Cut the sentence down until a friend outside your course would follow it, and add one comparison to something familiar. If the analogy breaks in an important place, say where, because knowing the limits of the comparison is usually the exam question.

The explanation you just wrote is card material. Each gap you hit becomes a quiz card asking the question you could not answer, and your simplified version becomes the answer in your own words. Cards made this way are short, because you have already thrown away the parts you were only copying.

Related: active recall and how to make good flashcards.

The technique is slow, so use it on the handful of topics you keep half-knowing: the concept that links several chapters, the mechanism you can name but not describe, the formula you apply without understanding. For pure facts and vocabulary, plain recall drills are faster.