After the border is cut off, there is a line all around the edges that isn't entirely clear like the rest of the bubble due to the natural imperfections of cutting it off and I matched that up with the heel cup and that's where you need to glue it back. I hope that makes sense. When you actually do it, you will know what I'm talking about.
I found that it's easier to just match the ankle bubble up with the back of the heel cup. It sticks out a little less than the other way I said how to do it, but the way I just told you is easier to do and has a better glue bond. And if someone here does do this mod, make sure to do a section at a time. The ankle bubble tends to pop out of place a lot if you try to glue it all at once.