Thanks! So for arms one of the things that helped me was to think of the arm muscles as links in a chain. The biceps and triceps attach around the deltoid, and the muscles of the lower arm attach on the side of the muscles of the upper arm.
I did a (very) quick drawing on my tablet that I think will show what I mean. It has helped me a lot with placing the big muscles of the arm.
It took me a long time to get to this point. About half of the art books that I own are about anatomy. One thing that really helped me a lot was a YouTube channel by Proko. His classes are really great and he covers every bit of human anatomy, its all free too.
Hope thats helpful!