In order to end up with a pattern like this for the front of the window at the bottom, required a slightly different approach, since I could not go outside to mark anything.
I wanted to end up with a pattern looking something like this.
Marking a rectangular piece of plywood, at the red circles, gave me a reference point to start the cutting process, but I needed to get a rough reference for the curve at the bottom.
That was accomplished by using what was handy, which in this case was a roll of tape. It's not an exact match to the curve, but it's close enough to get me started, and then I can finish the fine tuning after getting this close.
