LAWL SMEARGLE SKETCH GUIDE.
To start, let's say we'll want to give Smeargle Roar of Time (lol). Take a Dialga (that knows Roar of Time, of course) and a Smeargle, and put them into the first two spots in your party.
Now, go find a good place to do a Double Battle (I recommend going to the restaurant near Valor Lakefront), and start a Double Battle. Your two starting Pokemon should be Dialga and Smeargle.
Have Dialga use Roar of Time on one of your enemies, and then have Smeargle use Sketch on Dialga. Hopefully, Dialga will use Roar of Time, and Smeargle will Sketch immediately afterward. Finish the battle, and enjoy your new Roar of Time.
Basically:
1. Double Battle.
2. Copy Ally's move.
3. ???
4. Profit!