Before you decide to try to get a P-rank note that you WILL consider dying at some point while trying to get P-ranks

You should first practice by getting S-ranks, they helped me better understand the layout of levels
and they will for you too

Before or after obtaining an S-rank on the level you want to P-rank, just try to keep a combo for the entire level without doing anything else

Welp, that's pretty much it, this advice could be used for both levels and bossfights, also encase you didn't know if you press X then Down then you just found the quickest way to MACH 3