I first get the a recording and listen to it while reading the sheet music. I haven't seen many people here referring that, but I find that it helps me. I associated the sound with the note. Then, before going into the playing, or perhaps a bit as I make a first exploratory run, I divide the music in sections. Usually, you can understand different parts in it, even if it is small. Then, I make a plan to learn each section at a time, first hands separated, then hands together.
I do not worry about memorizing all at once. As I join hands, I make an effort to do it at speed and with musicality, even if I don't yet know it all by heart. As I play it, it will go in steadily.
Then, when I feel it is almost there, or that I am being slowed because I'm still reading it, I do without the sheet on purpose and go over the music again to learn it by heart. I insist on specific and short parts until they are learnt, and if it really is being difficult, I do it with the sheet in front of me.
Much of the work in memorizing is saved by the initial exploratory reading where I confirm that certain parts are mere repetitions of previous ones. I try to mark in the score any subtle deviation, or mark where some parts are the same figure as before, but some steps above, or below, or where some notes are the same and the others transposed. When I am trying to memorize, I already know where these tricks are and it saves time and effort.
Using all of these, I memorized Moskowski's Op 72 no2 in 4 weeks, with a minimal of effort (perhaps 2 days) to actually memorize individual parts. But I did a lot of work for short passages, like 4 bars at a time.
Alex