You'll have to remove a slice; if you just cut a slit, you'll have to overlap them, and you won't have enough seam allowance to connect them once you start getting to the point of the slit. If you cut a piece out, and have a backing piece of leather, you'll be able to sew all the way around.
This kind of stitching would be easiest on a post-bed roller foot machine. It's going to be a real challenge with a flat bad machine. Otherwise, I'd use contact cement to hold the backer on temporarily (or even just rubber cement), and then stitch by hand.
...And yet again, Gen X has been forgotten.
Cool cool.