Sandals for warmth

Step 5 Attach one side of the strap

Begin by doubling the thread over and passing that through the needle and knotting the 4 threads.  Then when you stitch you'll be using four threads at once.  Push through the back side of the webbing because the knot will probably not hold in the foam.  Use a whip stitch.  Keep the stitching low on the side of the sandal to allow for some flexibility of the strap.