Intro: Productivity is key. It does not matter if you think that sitting down to study piano for 2 hours is to study: IT IS NOT. Studying implies a progress, nothing assures you that sitting down to play some exercises will make you achieve that. Therefore, you must know what to do, to use that time productively.
1) Think before you play. Use the knowledge you are acquiring, including metrics, rhythm, tonality, texture, melodies, harmony and form. Visualize the piece in your head musically, then on the instrument. It is not necessary to play, you must have clearly defined your ideas first. Using what you know is a boost in your productivity.
2) In my experience, at least to read the piece once at an extremely slow tempo, without making mistakes, it is always good.
3) Divide and conquer: Separate by phrases, sections, the more you break the piece into small problems, and the solutions one by one, the more productive will be your time.
4) Connect the solved sections, connect them backwards (with the previous phrases or sections), always at an extremely slow tempo.
5) Do not trust yourself, playing something well once does not mean that your body has learned it, play it until you have that certainty. Not absorbing it involves the risk of making mistakes, once you make mistakes several times the body learns the error always reproducing it. * It is natural to make a mistake in a real performance (live), but (in the studio) playing a wrong note (or rhythm) because you did not know which one was correct is unacceptable. (The same applies to agogic, dynamic and articulations)
6) Do not waste time playing sections that you have already overcome. If your focus is productivity over time, then you can expect to have the piece dominated before enjoying it. If you want to be technically productive, studying is usually boring, tedious and often frustrating, do not be surprised by those emotions: master yourself and master the piece before frustration dominates you. It is very common to surprise (yourself) studying from past sections just to have a sensation of the beautiful or well that section sounds before the chaos; unfortunately if your approach is to solve problems productively: you must avoid it, focus on solving problems slowly and connect them with consecutive sections until you have mastered the whole piece.
Developing a little bit more this point: Learn to start the study from the sections and phrases that you analyzed (it is different to know it through the analysis than to study it from different points). 7) BONUS: once the piece is mastered in terms of muscle memory at an extremely slow tempo, enjoy it, sing it, begin to play it: you deserve it, you did it.
