Step 1: Materials
The pair I used happened to by 72" long, but I had lots of extra string left over. Fifty inch laces should be plenty long. Alternatively, measure and cut two pieces of rope - one 30" and one 50" (one of the laces will have to be trimmed anyway). Stay away from polypropylene ropes or anything else that is too slick as this sling relies on friction for readjustments.
Step 2: Tie Some Knots
1. Take one of your shoelaces and wrap the end of it around the yoga mat. The tie a slip knot--this allows the knot to slide and tighten the lace around the mat. You want the knot to be just tight enough so that it doesn't slip on its own. In the photo below, I have tied some variant of a slip knot. You can follow that or learn to tie a proper slip not here.
2. Repeat the process using the other lace.
Step 3: Tie Some More Knots
2. Take the free end of the other lace and string it through the loop that you just made. Fold the free end back down onto itself and tie another slip knot. This slip knot will allow you to adjust the length of the sling.