okay. It's been several eternities since I drew seriously, but I'll do my best.
As far as hands go, USE REFERENCES (you know that already), USE YOUR HANDS, and if at all possible GET SOMEONE TO SHOW YOU THEIR HANDS. hands are complicated
Foreshortening is, too. but it's basically seeing stuff in 3D and showing it in 2D. like all that is drawing, except
more. once again, references (especially live 3D references) are great for that (and closing one eye, or, a great technique I discovered is to hold something transparent like a plastic sheet in front of your 3D reference, so your brain instantly sees the stuff projected onto the flat surfece. That works really well for all the complicated volume stuff like foreshortening).
how you do it, besides looking at stuff, is to force yourself to imagine things seen at an angle (even stronger than you need, 'cause you'll probably do too little anyway. brains are weird). Your brain will tell you "no! that forearm should have a length, it can't almost disappear like that" Ignore that, it can. It gets hidden by itself and whatever things (even smaller like fingers) are supposed to be ahead of it.
Also, depending on how far the "camera" is, don't hesitate to strengthen the perspective and make whatever's close really bigger. BE DARING, basically.
case 0 is seen from above
case 1 is more from the front
case 2 has a bit of perspective and (should have) the hand's mass covering the forearm so that less of it is visible. The red circle-shape should actually overlap with the bar, but you get the idea
edit: I talked about forearms, but other things work the same, of course
edit2: and congrats on being paid to draw \o/