I had this problem, and I could figure it out. I wanted to teach mirror coat (a -5 priority move) to a male Smeargle to breed with another Pokemon in the Field egg group and pass it as an egg move (just because).
You need:
1. A fast Pokemon with Spore and False Swipe (Smeargle, Breloom)
2. A Smeargle for which you want to teach the move
3. A Pokemon that knows the low-priority move you want to sketch
Go find a wild Smeargle and have the fast Smeargle/Breloom as a lead.
Turn 01: Spore the wild Smeargle
Turn 02: Switch to the Pokemon that knows the low-priority move (If wild Smeargle awakes, leave and repeat until it doesn't)
Turn 03: Use the low-priority move (The wild Smeargle has to be sleeping; otherwise it will awake and use sketch, and you will have to start again).
Turn 04: Use the same low-priority move again. Wild Smeargle will wake and sketch the low-priority move you used the last turn.
Turn 05: Switch to your Smeargle, which you want to teach the low-priority move. The wild Smeargle will use the move you want to copy in this turn.
Turn 06: Use Sketch and copy the move. Done