In my past business I sewed patches on lots of leather jackets.
It's best just to use a standard machine, the ones with bells and whistles have no guts to handle leather. It isn't a hard job. I used a medium weight needle the larger ones will make bigger holes, not good, and tend to break. i know that sounds strange but it is true. I did use a little heavier weight thread, or the nylon is fine, just stitch along the inside
statin stitch of patch. Be careful of any lining, make sure it isn't twisted. Once you sew on leather thats it...so don't make any mistakes, like accidently sewing it on upside down or crooked, taking it off you'll have to match it back up so holes don't show. Believe me, customers and friends will frown on that..holes don't come out of leather. PLEASE DO NOT IRON PATCH ON....that's just digging your own grave!!! Hope this helped.
Good Luck!!