Bondaweb and supermend are two products that spring to mind (or else experiment with your fabric glue but spreading it very thinly), but I'd reccommend sewing your fabric down no matter what adhesive you use, as your customisation will be more durable that way.
If you want to make a template to help you cut the shape out, place a piece of light paper (I use baking paper because we always have some lying around!) into the leg of your shorts, and pin it to them along all the seams/edges you want your fabric to match. The idea is that when you take the pins out and remove the paper, you'll end up with lots of holes which outline the shape you want. You can then cut along these and transfer the shape to your fabric
Thanks so much!
I have that witchstitch stuff that seems similar to bondaweb. But it's thin strips for like hemming.
But it seems my best bet is to make a template. If I wanted to make the pattern slightly bigger than the shorts, so I could fold it over and get a clean line when I stitch how would I go about doing it?