Unlace sandal straps and remove buckle and keeper strap (you’ll reuse the keeper). Now you have a loose strap attached to the toe loop.

Be sure the strap is oriented flat, thread buckle on the strap so the tensioning side of the buckle faces the inside of the sandal. 

Insert the top side of the strap (dot side) facing away from the foot down through the hole at the outside of the foot. Adjust so your foot fits with just a bit of extra slack in the strap. As you tension the sandal, the slack will be taken out.

Lay strap flat across sandal in front of both side holes, dot side down

Wrap strap under and pull up through hole by instep of foot (dot side away from foot)

Take the strap running across the sandal and pull it toward the heel, orienting dot side away from foot. You’ve created your heel strap.

Get the heel adjusted now. It should be tight enough to keep your foot from sliding back on the sandal.

Thread the keeper strap onto the remaining strap, take strap and feed through main buckle adjuster and tuck tail through the keeper strap.

Make any fine-tuning fit adjustments as needed.

If needed, cut excess webbing and seal the ends with a lighter.