Use your fingers to pull the crack open slightly so that it will be easier to fill. Place your fingers around the crack and pull outward in each direction so that the crack widens slightly.
Squeeze a small amount of leather filler into the crack. Allow about 10 minutes of drying time. Use the hair dryer to speed up drying time if needed.
Cut a small piece of sub-patch to fit over the crack. It should be at least double the length and width of the crack.
Apply a thin layer of glue and then lay the patch over the crack. Press firmly for a few minutes to help the glue dry without letting the patch slip.