About drawing characters from behind and/or sideways, I guess you could think about what's hard about it to draw and figure out how to work through it
Use figures! 8D
If you're already good at drawing anatomy then maybe models or poses would help?
Learning anatomy can be very helpful. Or at least just looking at pictures in an anatomy book can help. I took an anatomy class so drawing characters at different angles and positions has become much easier. Especially when you learn about the skull if you're struggling with faces/heads
Well, for back views I've seen some stuff on drawing your character from the front and then erasing all the lines in the middle so you're only left with an outline, and then flipping it and drawing them from behind in the same pose. As for side views, I don't have much advice, mostly just studying anatomy? <X3 Sorry ;u; Best of luck with it though!