I like to drill my stitching holes with a 1/16” drill bit, using my drill press. I always lay a clean scrap of softwood lumber on the drill press table, to drill into. Among other things, this helps keep the leather cleaner, since my drill press table is often oily. If you don’t have a drill press, an electric hand drill can also do the job.
I prefer using a drill rather than just punching holes with an awl (which I have done as well) because, with the leather being fairly heavy, the drill bit gives a little better clearance for the needle while sewing.