I tried to keep the lacing instructions simple. Please refer to the pictures below for more detail!
First, prepare a 6' length of paracord (5' for children, as in this Instructable) by melting the ends slightly and pinching them while still hot, to facilitate putting them through the holes. Use pliers if you're prone to sizzling your fingers. Now put one end through the top of the toe hole and tie your favorite knot on the bottom (pictured is a figure eight).
Follow the pictures below to complete the lacing. Once you are done, complete these steps for the other sandal (look at the monitor through a mirror if you're directionally challenged!).