there should be a thin black plastic disc-like washer already on the yellow backing plate, around the threaded part that screws into the PC7424:
Your PC should have come with a smaller, thicker, grey rubber washer:
Put this washer on the backing plate as well before screwing it into the polisher.
Thinking like a sandwich, it would go: backing plate, black washer, grey washer, pc, with the screw part being the frilly toothpick that holds everything together.