To begin you need to make sure you match the correct strap with each flip flop. On the bottom of each strap, you'll see an R or L, R for right and L for left.
Insert the needle nose pliers as far into the toe hole as possible.
Open the pliers as wide as you can and then grab the toe strap and pull it through the hole.
If you can't pull it all the way through once it's at least half way through you can release it and then use the handle of the pliers to push it through the rest of the way from the top of the sandal.
Once it's pushed through, use the pliers to make sure it's pulled all the way through and then flatten it out.