First cut the feet off of a pair of pajamas. Leave several inches at the top, so it looks like a boot.
Sew (or glue) velcro to the strips. Make sure that you are attaching the velcro to opposite sides of the fabric. Put the loop velcro on the front of your strip and the hook velcro on the back of your fabric.
If one of the legs of your slipper has a zipper in it, stitch it closed.
Attach the strip to the back of the slipper.
Also--now that you have your materials out, you should get out your child's mittens and make strips for those too! I did, and it would have been awesome, but 1) they were too long and 2) the glue did not hold because of all the stretching. But once my sewing machine is back in business I will definitely make some ;)
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