You can actually stop here and have a neat pair of shoes. Instead of pony beads you can use any combination of moderately sized beads for the straps. You might want to use smooth beads rather than facetted ones in order to keep the facets from irritating the skin on your feet. You can also add a bit of bead weaving detail to the finished shoes.
Thread a needle and enter the left hand strand of the pair on the right by going up through a bead about 3/4 of an inch from the sole. Do not tie off; instead leave a long tail and be careful not to pull it through with the first few stitches. Then pick up 3 seed beads and go up through the bead 2 beads up on the right hand strand. That will make a diagonal line of seed beads from one side to the other. Pick up 3 more beads and go up through the second bead up from the bead you originally exited on the left hand strand. Continue adding beads in this zig-zag fashion until you get to within a couple of beads of where the straps join. Thread back through the whole left hand strand and tie the two ends of the thread together and hide the tails in the main strands.
You'll love your new shoes! There are many other things you could do with them. Wouldn't freeform straps be neat? Or a peyote tube for the "in between the toe" part then flat peyote straps? You might need to back those though. Most of all, have fun with these Beaded Beach Shoes!