If you work from printed music you have many opportunities to make notes. Use a pencil – you might want to come back to the tune and do something completely different in the future. Mark all the challenges you meet – fingerings, lever (or pedal) changes, dynamics, expression and phrasing – all of these will help you work through the music and will also serve as documentation of your ability to play it when you were working on it. Always use a pencil so you can make changes as you become more capable and experienced. For instance you could start a practice and performance journal.
No matter how you learn your music, make yourself a journal. Write yourself notes about your practice. Write down what is going wrong. Note what have you tried. Jot down what went right. Be sure to include notes about what do you want to try as well as writing out how are you going to go about learning these new things. Also note things you still want to learn.