> take every chapter of the textbook and rewrite it, filling in all the intermediate steps of every proof, those where the author writes "it follows that ..." or "from which it's obvious that ..."

I recently-ish had a read through some of my old fundamental/pure maths notes from Uni, including plenty of proofs. The damned things are littered with steps which were "obvious" to my smug self-satisfied 20 year old self but impenetrable to me reading without much context 20 years later.

Git.