Notes on slips of the tongue, deliberate practice and learning things the slow way.
After about forty minutes of focused practice the error rate creeps up in a very particular way: not random noise, but the same two or three slips repeating. I started logging them in a plain text file, one line per slip, and the pattern became obvious within a week.
Flashcard apps are built for facts, but the same scheduling idea works surprisingly well for scales, typing drills and handwriting. The trick is to rate yourself on the first attempt only.
Three books I keep going back to, with a paragraph each on what actually changed in how I practise.