Ingredients: Humourous stubby holders (assorted); scissors.
Step 1: Cut down side seam (or for bottle holders, along each side of the zipper).
Step 2: Unpick or cut around base of stubby holder. Mine were glued under the stitching, so I wound up just cutting them.
Step 3: Voila - one heel cup. The vertical neoprene is attached around half the circumference of the base. The flaps just fly free.
Step 4: Insert into back of skate. I've put this one in inside out, cos the colour makes it easier to see, but it makes a little lump at the base of the heel. This may or may not matter to you - I'm putting an insole over the top which covers that edge.
Action shot!
Not pictured: Step 5: Trim to not look stupid.