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[Hide] (78.5KB, 1000x881) >>2653
It takes a long time to get familiar with all the shapes in a body. I think it's a skill that's very hard to practice in specific, because there's so much information. It's something you learn and hone over a long period as you keep drawing.
For example you may know that an arm gets thinner towards the wrist. But then you may learn that the wrist is also flat. And then that the arm isn't consistently shaped because muscles bulge in different positions on different sides of the arm. The elbow bulges out when the arm is bent, but bulges sideways and almost sinks inwards when the arm is straight. There's an inset on the inside of the elbow joint. The shoulder tapers into the arm around the same height and behind where the topmost muscle starts bulging out…
You keep learning more details on top of the previous knowledge over time, but there's just too much to learn in one go, so you should take it one step at a time. Don't expect to learn it anytime soon. Draw whatever blob you can and then keep trying again using references, draw close up shots for more detail. You'll keep learning small bits and pieces that add up into a pile, and when you draw a lot you don't even have to really try to remember it because you know what should be there. If you try to remember the whole anatomy book, it'll be an overwhelming amount of information and you'll just forget most of it.
TL;DR: patience. Loads and loads of patience.
Also, try to hunt more specific problems. Don't just draw an arm because you want to learn to draw arms. Try to figure out what in specific is giving you the most trouble, and then learn to improve that. Maybe it's the way the shoulder connects to the body, maybe it's the way the arm connects to the shoulder, maybe the wrist shape makes no sense, maybe you don't know where the leg muscles are, maybe you don't know how the collarbone works…
I would guess that your most prominent problem in the beginning is simply proportions. How wide are the shoulders, how far down the arm reaches, where the elbow is, how long and thick the neck is, where the hips are, etc.
So for now, try to draw random thick figures (get it? thick figure? stick figure? thick stick figure? I'm either the most clever or most annoying person) and try to give them the right proportions. Remember that the width of each shape is almost as important as their length, for example you can make the top of the upper body wider, make the waist thinner, or make the arms attach farther away.
Pic related is a shitty example.
Also, don't be afraid to "cheat". If the legs look too short, just cut them and move them downwards and fill the gaps. If the hands look too wide, cut half of them and move it closer to the other half.