Like any piece. You identify the difficulties, you break them down into manageable pieces, and work on them until you've mastered them. If you struggle with the polyrhythms, identify which polyrhythms are present in the piece. For example, (inventing now), there might be a couple of 5 against 4,and a couple of 3 against 2, on top of which is a 4 (sixteenths for example), which consists of 3 sixtenths, a 32nd-note rest, and a 32nd note. Then you take these two patterns and study them until you've mastered the difficulty. Then all these places in the piece are essentially solved because you know how to tackle them.
I'd argue the no. 9 "Black Mass" is the easiest of the late sonatas, if you've reached the technical level needed. I think you might find that many parts of that sonata are quite playable, it's just mixed with more difficult stuff.
(I've learned no 2, and worked on no 5 and no 9 but did not finish them because I got distracted by other music. No. 5 is a bucket list piece for me).