What is the best way to trim the top of a half wall in my basement.

I have bullnose corners and many height changes where the windows are to where the helf wall is. And one bumpout for a drain.

How do I decide where to cut the oak around the bullnose so it fits well.

is there a template somewhere on how to cut it? or do I make a guide? I have one piece that will need to go around 4 bullnose corners.

I can get pictures if needed.