Well for just adjusting it, you should do it slowly, avoid abrupt reductions (i.e. don't pull the mixture straight to half-way out, in flight.)

If your asking on how to best lean it, it all depends on what type of performance you want, best range or best endurance.

Anyway, the best place to find the most accurate/correct way to do it is in the Cessna 172 P.O.H. it should explain how to properly lean for both taxi and flight, and what you want the engine instruments to be reading for the type of performance you want.