Yeah, the whole sandal is crocheted, including the single crochet sole! An ankle crochet chain strap keeps the crochet motif and heel secure.
This twine is very inexpensive. You can purchase it here.
Notions: St marker (m); one pair of inexpensive flip-flops in proper size with ½” uniformly thick soles and straps removed, in a color similar to twine; sewing pins; metal yarn needle; one 2.8 fl oz tube of 100% silicone rubber sealant; latex or nitrile gloves; adhesive spreader for vinyl flooring (available at hardware stores in the flooring section-a piece of plastic with a handle on one end and a serrated edge on the other. It is used to spread the rubber sealant evenly onto the bottom of the shoe. I used Homax brand 3” spreader with a 3⁄16” tooth).