Okay, where do I begin? You might think that what makes a woman sexy is an anime face. You'd be wrong.
So then, it might take her to have big... "attributes". Wrong again.
How about some delicate curves? Yeah easier said than done.
If you ever try to imitate someone else's drawing of a woman, you'll come up with a transvestite. That's completely normal, drawing women is hard as shit.
Start with something supremely basic, you don't even have to draw a pretty face, in fact, you might not need to draw a face at all:
Look up bases for sexy poses, better yet, try using pictures of naked models for drawing, and, without tracing (EVER!) try to imitate those poses, watch the delicate curves and the balance of the figure, and try to replicate that, always respecting geometry and anatomy, over and over and over. Untill doing a sexy sillouette becomes second nature.
The rest, with practice, comes on it's own. If you end up making an ugly lady, her delicate pose will make her feminine enough, so fixing her face will be far easier.
Can't stress enough how important is anatomy and geometry here, NEVER skip them.
This is what worked for me, it might not work for everybody, but if you're out of ideas, there's a start.