If I'm remembering correctly, I initially used youtube videos to get the hang of using a round knife, but one thing I got from one of the Stohlman books was in regards to push cutting tight radii. I've definitely not perfected it (advice welcome), but the idea is to rock the handle down towards the surface of the leather, even touching it. This will probably work very differently depending on the knife, of course, but it was a help for me.
Also, might want to avoid using one of those green rotary knife cutting mats. They can snap the tips off a knife with a very acute point. I haven't had it happen personally, but the tip has certainly caught in the mat while cutting a tight corner. Best to use poly, uhmw (or similar) or a clicker surface.