So I'm very new to upholstery and I'm working on a leather motorcycle seat that I want to replace the old insert on the driver and passenger part of the seat. I want to use the original leather that is on the seat and still in good condition. How do I sew the insert to the original leather and match up the original stitch marks so that it doesn't look sloppy on the leather?
If i understand correctly you want to use the original outer leather band and sew in a new insert.
So if your using the original insert as a pattern you don't really need to change anything on the insert.
Just sew the outer band 1/16 or 1/8 past the original sew line. That tiny bit is not enough to make a large difference when putting the cover back on.
If the outer band had a top stitch i would still sew past the top stitch holes about a1/16
So in if the seam allowance was 3/8 now its 1/2 inch.
You could add 1/8 to the insert but not necessary.