how do you repair a puncture in leather car seats?

Answer 1

There are repair kits on the market. I once bought one through the J C Whitney auto parts catalog. Furniture stores where leather furniture is sold may also have repair kits for sale or may know who in the area does repairs.
Check with a claims adjuster from your insurance company. He may know who does that type of repair.
I have never seen a leather repair that looked acceptable. So don't expect much.

Answer 2

Duct Tape. :) There are some expensive repair kits out there (first answer) so you can give that a try but I've never seen anything in a store - probably have to order it online.