Next, we're going to carefully cut through the top layer of foam and remove the resulting disc

Use a craft knife to carefully cut along the circular line, to a depth of 2-3mm. (red arrows)

Take care! The objective is to only cut through the first 1/4 of the foam thickness. cutting too far will weaken the strength of the flipflop.

Next, very carefully, insert the craft knife into the middle of the hole, and cut horizontally through from the hole until you connect to the circular cut made already. (green arrow)

Use a circular cutting motion to slice the knife blade through the foam, using the hole as the central location. (yellow arrows)