There's a rubber membrane with a small hole in it, stretched across the blue circular frame. Once you get the rubber membrane around your cast at the toes, it's tempting to simply pull it the rest of the way on by pulling the blue frame. This is fine once you are past the heel and you are pulling along the length of your leg. But it is not okay while you are pulling around the corner of your heel. By pulling on the frame alone, you risk pulling the rubber membrane out of the blue frame. (If you pull hard enough, it *will* come out.)
The correct way to put it on is to use your fingers to hold open the rubber membrane hole as you navigate around the heel. (A third hand really helps.) If you do it this way, you will never pull the membrane out of the blue frame, and it will outlast your cast.