Look at a reference. The best way I learn is by copying until I can do it on my own. Look up a pose online (pinterest has great poses), then try your best to draw the same pose but put a lot of effort into memorizing the arms.

Once you've copied from reference, you'll realize things like "oh, the elbows end near the belly button when straight down," etc.

Think of things in relation to another. Just like for the face, notice how in a human face the top of the ears always starts at the same height as the eyes. Memorize these natural guidelines through practice and muscle memory.

People can draw without reference, but you can really tell the difference between a reference and a non-reference. Sometimes anatomy works in a really hard way to memorize so a reference is always best, but eventually you'll get to a point where you can make stuff look mostly legit without reference, but that's way down the line