In DPP: Below the Pokemon Mansion by a bit, you will see a large patch of grass. In the northern tiles of this large grass patch, use poke radar. There is a 22% chance he will appear. He appears in the morning, day, and night. You get the poke radar after getting national pokedex.
BW 2: Route 5
Wellhe learns sketch at levels: 1,11,21,31,41,51... every sketch you use smeargle will get that move to himself. then you either give the exp share or use your new move to train him up. Every ten levels as shown above he learns sketch. MAKE SURE THAT YOU PLAN A MOVESET FIRST.