For sketching a move, people usually bring the smeargle(holding a lagging tail is recommended) and the pokemon knowing the move and hunt for a double battle in friend safaris. In a double battle, use the move, then sketch it with smeargle. Lagging tail ensures smeargle moves after the other pokemon so sketch doesn’t fail.
You can also go to a smeargle friend safari, lead with the pokemon knowing the move, use the move, opponent smeargle sketches it, switch in your smeargle, use sketch. This way is probably faster than searching for double battles. (Credit for this idea: @Wwcheezburger)
Repeat for grass knot after leveling up smeargle(it learns sketch every 10 levels) or using a heart scale to relearn sketch. Giving the smeargle a lagging tail and giving the other pokemon a quick claw helps.