Easiest in my opinion is using chemical guys cleaner/conditioner combo.
1. vaccum seats thouroughly
2. spray cleaner (if using CG, I use at 6:1) directly on seat
3. Aggitate with leather brush
4. Wipe away with microfiber towel
5. Make sure seat is dry then apply conditoner with applicator and work in
6. Buff off excess with a clean microfiber.