David,
I get my leather wet before I stitch it on, and then pull it tight with the stitching. when it dries the leather shrinks tight and conforms to the curves. you have to make sure you have just enough slack in your seam that the drying process doesn't create a gap, but still tight enough that the leather conforms to your curves. It isn't as hard as it sounds, though. I usually make a pattern by wrapping with masking tape--put some paper on the back of the bow first, then wrap with tape and then cut a straight line down the back over the paper and unwrap. That pattern plus about .25" to account for the thickness of the leather usually works about right.
Clear as mud?