You have to understand what’s on the inside to do a good job on the outside.
What I did is I went ahead and got some tracing paper from Michaels, although you can find it anywhere, and searched up the human skeleton. I drew it. Then I put another paper over it and drew all the muscles. In the end, the last tracing paper was for the skin. I added all the different dimensions to it that I could see underneath from the muscle arrangement. It looked great. Now that I saw the overall anatomy, I took my time familiarizing myself to all the different muscles and joints. I googled them and went to town, drawing and drawing, until I filled up the entire 25 pages of tracing paper. Now, this took a long time, not just two days, and you don’t have to take that long of course, depending on how much free time and how dedicated you are to learning it, but this exercise of layering the body really helped me.