Clean the rip with an abrasive pad. Be sure to wear down both edges of the rip to a smooth surface.
Scrub with a gentle solvent to remove any dirt or oils and make the surface as smooth as possible.
Rub in a leather glue substance if it is a smaller rip and allow it to dry until it seals. You can purchase this glue from furniture stores or with leather repair kits. It is made of a substance compatible with leather surfaces, and will not unintentionally stick. Use a smooth pad to rub the glue onto and over the scratch. Be sure to cover the length of the scratch and gently rub the glue in until it is smooth against the surface. Wipe away all surface glue when you are finished. The glue should remain only between the rip.
Cut a piece of similar leather just larger than the surface and place it underneath the rip for larger surfaces.
Use the same rubbing method to glue both sides to the top of the underneath piece. Use your abrasive pad to smooth out any differences after the piece has dried.