To make space for the plastic washer, we need to cut away some of the thickness from the disc

Cut away approximately 1mm of foam from the backside (inside) of the foam.

Tip: Through the process of removing the foam, keep placing the washer into the hole in the flipflop, and cover with the disc. Keep removing material from the disc until it is flush with the bottom of the flipflop when the plastic washer is installed too.