funny you must reeeally like her ass to feature it so!!
My suggestion would be to get some books on human anatomy and proportion, ones with titles like "human anatomy for the artist" are good and available in university libraries everywhere... you can scan them in and try modelling around them with a drawing setup like raven said somewhere above and then when you know what goes where you can start messing with it, have to know the rules to break 'em...
But like I said before, it's nice to see a female model with appropriate breasts, not ridiculous watermelons hanging off a twig... I'm guessing you have a life outside your computer...
ZX Spectrum 48k Issue 3, Radio shack Tape drive, Rank arena 12" T.V. set.