You might want to specify Sigma 6 or 3.75 inch figures, because it's probably going to be different for each.
I can maybe give you a Sigma 6 POV:
Don't really know how much I can help you...but mainly I just work on the putty until I'm happy with how it looks. Obviously females have more "chest" than the guys...you just have to decide if you want a Barbie or something more realistic. On my first Scarlett I went lower key cuz it's for my kid to play with and he doesn't really care if she's well endowed or not. I probably could have even gone less and he wouldn't have noticed. My second was for someone else and so I was a little more generous...lol. Personally, I think with the extra up front, the girls also need a little extra on the behind...you gotta admit these guys are pretty flat-butted! So I just add some to round it out a little. I also add a little to the sides of the hips and torso to give a hint of curve. Again, I'm not going for the hottie look...just enough to make sure people know she's a she.
I painted a Scarlett designed by a good friend of mine, and his sculpting is very low key...definitely female, but more athletic than beauty queen. I don't know if i posted that one up here or not, but you can take a look at my customs to see what I'm talking about. The hardest part of the sculpting is getting the boobs even. But you just get the basic form on there, and shape it and smooth it so they're basically the same shape and size. Then I let the putty harden and dremel and sand to get them right. Apart from the sculpting I take a Dremel to the hard angles of the musculature...especially the lower legs, and some of the upper arms. If you don't then I think the legs especially look out of place. Lots of guys approach the females differently, but for me I insist on using all GI Joe parts for my bodies. Maybe that limits me, maybe it challenges me...it's all POV. Hope you get something useful out of that!