Test the color selections that came with the kit for the best fit. Dab a dot on in an inconspicuous place and put a good light on it after it dries to see how it matches. Pick the closest color for your repair. Don't expect a perfect match unless you are fixing a black seat.
Cut a piece of iron-on patch from a fabric store. Make the piece small and push it through the split. Unfurl it and position it on the backside of the slash. This will further enhance the bonding agent's ability to pull the slash together.
Apply the color of bonding agent that you chose, using the applicator. Paint it on the split, and feather it out a bit so the area blends better. Plug in an iron, and set it to medium high. Cut a piece of bonding paper to fit the split, and place it over the hole. The bonding paper comes in different grains, so pick one that most closely matches your leather seat.
Use the iron to heat the metal transfer tool, if your kit has one. With some kits you actually iron the bonding paper, but the transfer tool provides just enough heat to seal the bonding agent and paper. The heat goes through the leather enough to adhere the patch on the back side, which makes the repair more secure.
Wipe on a leather cleaner and conditioner to protect the leather. Moist leather is less likely to split and crack. Condition the seat once a month if it is exposed to sun or outdoor conditions.