I've seen a few how-to vids on this subject and I find this one to be pretty good. Notice min 2:10 on video 3. This is critical to make the string follow between the 1st/2nd metatarsal. Crossing the long 2nd metatarsal bone is where I experience chafing.
This guy was clever and converted some cheap flip-flops. (although the sole material is probaly overly cushioned, it's a thought: