Measure out the lace to cover one shoe, but don’t cut it. Now using the hot glue gun glue the lace around the bottom of then shoe.
Now you can cut the lace, make sure you leave enough to hang over and inside the shoe. Using a needle and thread, sew the lace around the top of your flat.
If need be, sew another line of stitching inside the shoe to stop it from flopping out of the top.
Repeat for other flat.