Scissors, preferably the pointy ended kind.
Hole punch, if there are no pointy scissors.
String, ribbon, or narrow elastic.
The Making: Start with a large piece of cardboard. Put your foot on it. Put your weight on your foot and make sure you are standing up. Then, take the pencil, you can have a buddy do this for you if you can, it's easier, and draw around your foot. Make sure to keep the pencil STRAIGHT UP at all times. I cannot stress how important that is. Otherwise the shoe will not fit right. When you have done one foot, do the other, on another piece of cardboard. When you have this, cut the feet out of the cardboard. If your drawing has the end bumpy from the toes, you can smooth it out. Having done this, you have to soles of your sandals.
*Put your foot on one of the sandals. Take your pencil and put a dot near the middle of the shoe, about where flip flop straps come to. Do the same on the other side of the shoe. Then, put a dot between your big toe and the one next to it. Finally, put a dot at the heel. Then take your pointy scissors or hole punch and make a hole where each of the dots is. Next up, our lovely friend, scotch tape. Take a piece and put it between the hole and the edge of the shoe, front and back, for all of the holes except the front one.
Having done this, you can begin to attach the straps. Put your foot back on the prepared sole, and measure the string. Take two to three inches of string and put it off the side of the shoe, at the latitude of the side holes. Then unwind more string across your foot until it is comfortable, making sure that the couple inches off the side stays there. The strap should be loose, but not too loose. When you have the tightness you want, unwind another couple inches on the other side and cut. My middle strap was between five and seven inches long. Then you tie one end through the hole on one side, and put your foot back on if you took it off. Then pull the strap across to a comfortable tightness and tie it on the other side. Cut another piece of string maybe an inch longer than the first one and tie it through the front hole. Then you tie that string to the middle of the first strap. There should be several extra inches. DON'T CUT IT OFF! It will be used in the final part of the making. The last strap is simple. Cut about a ten-inch piece of string and tie it through the back hole.
Now, if you want to, you can put strips of duct tape on the bottom. I recommend it. Cardboard is very slick and can slip you up. If you put duct tape on it, it will help with the slipperiness and it helps secure the ends of the straps, as well as reinforcing the whole shoe.
Repeat from * to make the remaining shoe. To put on the shoe, you slip your foot into it like a flip flop, but then you take the back strap and wind it around your ankle once, then tie it to the extra from the front strap.
Again, from the same website, Valenta also said:
Decorating your flip-flops is a good way of showing your own style. I bet you can think up a lot of good ideas yourself! But just to get you started...
Wind your flip flop straps with strips of fabric or ribbon. When you get to the end, you can hot glue the ends to keep if from unravelling, or knot it around the strap. It's a good way to make them look really great, and it will stop your feet from sticking to the plastic straps.
Put stickers all over the flip-flops, especially the straps. They can be sparkly, plain, made in the shape of horses or stars to roses and pigs. Just see what you've got around.
Water balloon flip-flops! Get a pack of water balloons, plenty of them. Then, tie the balloons around the flip flop strap. When you have done the whole strap, your flip flops should have a bushy cover of balloon ends that looks flowery. The inside should still be comfortable, as the ends are on the outside. This kind of flip flop, unlike the rest of them, is waterproof. For a video of this, go to www.youtube.com/watch?v=rD6G2Qs6HSs
Crochet around your flip flop straps. For how to do this, visit www.youtube.com/watch?v=T_Rz2MXc6-Y You can also use this method to crochet around headbands.
Use your imagination! Find scraps of tinfoil or paper or cellophane and cut shapes out of them, then hot glue or waterproof glue attach them on to your flip flops. Tie ribbon bows on the straps. You are the designer! Do what you want!