I'll be the first to admit that I don't draw a whole lot of women. That being said, I've really been motivated to draw them more after finding female characters that I actually like or relate to specifically from the shows: Venture Bros and Game of Thrones.
A bit of a forewarning below.
Most of these studies were referenced from my own body. If you don't want to "see me nude" then please just skip this post entirely. Thanks! :)
Side note, any boob size that's smaller than a "D"ish cup isn't me, lol.
These top studies were me just trying to figure out how boobs worked in general. The bottom ones are more organic as I later figured out how to better balance the weight depending on the body's movements.
I'll be doing a ton more studies of boobs 'cause I want to and need to learn more about them and because they're interesting actually! I learned a lot in the half hour that I did these from pure observation and this goes to show that sometimes the best learning tool isn't a tutorial or book, but your own body. Besides, there's been no boob tutorial that's helped me in any way, so these were all how I came to figure out things and they appear to work for most breasts.