Cheryl purchased various colors of foam sheets from Michaels.  After tracing Matt's feet, she cut out the sole of the flip flops.  Then, she cut out small "strap" which were a bit longer than the sole so that she could turn the ends under and sew them to the sole.  She used thread matching the color of the foam sheets.

For variety, Cheryl made the straps in a few different ways: 1) a single strap across the foot (width of strap can vary); 2) parallel straps - using two to three straps; and 3) 2 straps which criss cross in the middle.  For the criss-cross straps, Cheryl puts an extra stitch in the middle to secure them in place.

To make the frog feet, when tracing Matt's foot, Cheryl just added onto the sole by drawing free hand what she thought a frog flipper would look like.

To get fancy, Cheryl suggests making the flip flops and frog feet out of leather or other materials and then adding a cover to the top of the soles which would cover up the bottom of the straps and/or a sole on the bottom to hide the stitching.