Glue in a bit of cloth over the worn out part of the heel. Remove the insert first. Then use a glue that is flexible to fasten the cloth into the heel. I used glue that was designed for gluing soles together. The choice of cloth is important. It must be durable but not too thin or too thick. I used old denim from a worn out pair of jeans.
Step 2: Stitch in Top Edge
I think the top edge is the most likely place to fail, so I stitched that in. I used a curved needle, but it's not necessary. Use a thin sharp needle and some strong thread.
Step 3: Finish
Put the insert back in, trim ends of thread, and take a step back to admire your work.