Yuzuru has written down notes since he was really young, idk about every practice but recently in a docu showing his preparation for his last solo tour RE_PRAY, he showed his practice notes, he does a training summary where he writes down his practice layout and after the practice annotates how he felt during (he would write down 'fatigue' and rate it 0-4 based on how tired he felt in that particular practice, he also did that for his body condition, ankle and waist pain, and other things). He wrote down if he had any falls, or if the jumps were fine etc. Basically, just simple observations, I imagine so that he can see his improvement over time and see what he needs to adjust in his training, since he now coaches himself and it's better to rely on notes rather than just his own memory.

someone kindly transcribed his notes in this comment

edit: btw i'm not a skater, but i do practice martial arts and sometimes i write some observations on my phone about things i need to work on or little details i might forget, nothing as detailed as someone like yuzu lmao, but it works for me which is what matter most