Thank you so much!
For this one I drew up a template in inkscape. It's a free vector drawing software. And then I traced it onto the leather and cut it out with a head knife. You could also use a small utility knife as well.
Another low tech way to do it is to find something round (think jar lid, or LP record, etc...) mark out where you want the curve to end and then put your round object so it matches up with the two marks you made.
I hope that makes sense...if it doesn't tell and I'll try to explain it better.