First are the materials necessary. You will need:
-a pair of [old?] flip flops (I close the pink ones from the photo)
-a pair of long shoelaces (I chose the dollar-store white ones with colourful dots)
-some clamps (like the ones used to hold laundry on wires to dry)
-glue (I'm not sure which works best. I use an adhesive which claims to work on plastic)

First, I started by gluing the shoelaces along the flip-flop straps. I didn't start right at the centre, I left some extra length on the side of the flip flop which would be facing outside (i.e. for a flip flop on the right foot, i left extra length on the right side. You will see why later)
One the V in the middle of the flip flop, i had to cut the shoelace so that it would bend as needed.
Do this for both flip flops.

As I glued the shoelaces, I clamped them to keep them on.

When they are done, they should look something like this.

When finished, I put clamps all over and left them to let the glue dry.

When they are dry, put them on and wrap the laces around your ankle,you can make a knot [if you want] at the back, so they always stay in position]

Next, tie them up in a bow on the outside [i.e. on the right foot, tie them on the right side] of the foot. Voilà!