really does depend on the moves you want. but my experience has shown that you want a pokemon that has the move you're looking for, and then either level that one up higher than smeargle, put the choice item that boosts speed on it, or put an iron ball on smeargle just to make sure he's slower and then go into the grass that can be double battles. then you use the move you want and use sketch.
if you want a lowered priority move you apparently need to get a pokemon that has the move, can learn rest, and can use sleep talk. then you use sleep talk to make the lowered priority move into the same priority bracket as sketch. make sure it only knows sleep talk/rest/move you want. this may take a few attempts which kinda sucks because for each fail you need to lvl smeargle to the next x1 level to learn a new sketch.