Choose the right leather needle
Use strong thread such as waxed polyester or nylon
Mark the stitch line with a ruler or scratch awl
Punch or pre-punch holes with an awl, chisel, or stitching punch
Clamp the leather in a stitching pony or between clips
Cut thread to a suitable length and wax it if needed
Thread both ends for saddle stitching if hand sewing
Start from the back side and keep stitches even
Pull each stitch snug without tearing the leather
Backstitch a few holes at the start and end to lock the seam
Trim excess thread close to the surface
Tap the seam flat with a mallet if desired
Use a leather sewing machine with the correct needle and presser foot if machine sewing
Test on scrap leather before sewing the final piece