You may find sites that give hints for sewing stuffed teddy bears may have help for this too. I have sewn with leather (for the above mentioned teddy bears). There are a few weird things - like leather is not going to "slip" through easily. You know how cotton sort of glides along as you sew - leather not so much. I have had decent luck putting a piece of freezer paper over the leather (both sides) and sewing through it. The paper helps the leather "slide" along. Definitely slow the machine down. Use a leather needle - the tip is shaped differently to help pierce the leather.
When working with leather there is no real do-overs.