Okay first you need a Pokémon that knows Baton Pass and is faster than Smeargale. If you have a Baton Passer but it isn't as fast as Smeargale you can give it a Choice Scarf to increase it's speed.
Step 1: Compete in a Double Battle with Smeargle and another Pokémon that knows Baton Pass as your two Pokemon. Note: DO NOT have any other Pokémon in your party except Smeargle and your Pokémon that knows Baton Pass.
Step 2: Have your Pokémon that knows Baton Pass, use Baton Pass and have Smeargale use Sketch on the Pokémon that has Baton Pass.
Step 3: Finish the Double Battle and Smeargale with have Baton Pass.
Hope I helped :)