1. Choose at the start a side of the boss to do the simulator in.
  2. Your debuff color will show on the top-right corner, and click on the box with the correct paw.
  3. If you succeed, you can click Next to go to the next debuff phase. If you fail, restart and click a side again.

Remember: The order of the Black/Orange zones on Vergo will alternate. Normal > Opposite > Normal > Opposite > etc.
See Phase 4 section of Harrowhold 20-man guide for more details.