Perspective can be very hard. But also very awarding when it is right. I would recommend drawing lines to help. The light source is also very important To create a less flat look and create the mood.

I did a fast googling and this article seems good https://mymodernmet.com/perspective-drawing/

It also talks about the shades needed to make it seem less flat.

I would also recommend using your own body by taking a picture of yourself posing. You will easier see how some parts looks bigger and smaller depending on the perspective. Pinterest is also a good source to find ppl posing. And there is an deviant art account creating a lot of images of their team posing (even action proses with props). Good luck!