Remember to soften up those new sanding blocks on a piece of wood. The corner edges can be quite sharp when new.
You need to just key the leather but where the creases are you can push a little harder.
I actually sand quite aggressively in some places but this comes with experience and I don't recommend you do this on your first go.
You want 99.9% alcohol if you don't have any. Like this one
I can thoroughly recommend going to u tube and finding the furniture clinic channel, it takes you through Al types of seat repairs, I personally will be taking of the plastic film on the leather, re colour, then reapply the protective covering, and will use the furniture clinic air brush to give a uniform and lasting factory finish............I've probably opened up a whole can of worms now....