Use a stitching groover to create the groove where you’ll sew. Just “slide” it along the edge. You won’t have to groove the side where the spine will be attached. We’ll put some leather on top of that side.
This groove is another small detail that will help set your case apart and it also protects the stitching to make sure that your case will last 20 years even if you’ll buy a new Kindle next year!