Not everyone's feet are the same size so i cannot provide exact measurements. This is pretty much a preference type thing. The use of your flip flops depends on what materials you use. I recommend using cardboard to support the bottom if you plan on wearing them around (which if you make them i presume you do) NOTE: I will add more pics when i have a chance ( probably of them with the carboard on the bottom) and ignore my icky feet lol
Take the roll of duct tape and measure the length of the shoe with it ( so you have flip flops long enough for your feet) Just remember not to measure with the sticky side facing your shoe ( you don't want it top stick to your shoe)
Okay how many strips of tape you use is based on how wide your foot is. For me i made my sheet of duct tape 3 pieces wide. ( to make a sheet you just take your 3 pieces of duct tape ( that are all the length of your shoe) and slightly overlap the edges then cut 3 more pieces the same length and put them on top of the pieces you just put together ( sticky sides will be together). It is best if your not that great with duct tape to do it one strip at a time so you don't mess up. If you have a bit of stickiness on the outside its okay it will be fixed later. ( you will need 2 of these, one for each flip flop)
Ok i DID have pics of this when it was a complete sheet but they have disappeared so make sure you put tape on top of it ( sticky side down)
Alright now take your shoe and sit it on top of your duct tape sheet.
You will now take your pen and trace an outline of the shoe. For best results hold your shoe in place so it doesn't move and mess up your outline. If you fudge it up the first try remember there is another side you can try it on.
After you trace your shape take your scissors and cut out the shape. ( At this step i usually put my foot on it to see how its gonna fit. You can always cut off excess if needed)
After, take the bottom you just made and trace it on your other sheet of duct tape ( this ensures they are both the same).
NOTE: remember when you go on to the next step that you don't want both of your bottoms facing the same way ( so you don't have 2 rights or 2 lefts) so make sure you have a right and a left.
For this part you may want to refer to a pair of flip flops. For best result i would take a piece of duct tape ( this is going to be the part you put between your toes) and cut it in half. ( save the other half you will need it as well) For both halve you will want to fold them until they are thin then set them aside. ( if possible leave some sticky at the end to attach them to your flip flops, if not you can always take more tape to cover them up.)place your foot on the duct tape and make a mark right between your big toe and second toe
( do this for both of your flip flops).
Now take your pen and poke a hole in place of the dot you made.
take the 2 pieces of tape you folded and thread them through the bottom to the top. ( for this part i used my feet again to measure how far back i would place and take my strips)
After you get them to where you want them use a small piece of tape to keep them in place, then tape them onto the bottom of the flipflop as well.
( Repeat process with other flip flop too.)
If you aren't stabilizing the bottom this is the end for you. NOTE: next 2 steps are completely optional ( although step 7 is recommended)
Place one flip flop on a sheet of foam ( something like a memory foam type material) and trace it, then cut it out. Make sure you cut out 2 of the same size. To attach the foam you can either tape the sides of the foam to the sides of the flip flop or fold over a few small strips of duct tape to make them double stick and just tape it right onto the bottom of the flip flop. Either way will work.
Take some Cardboard and trace the shape of the flip flop on it. Yet again we are tracing the shape. Cut out 2 of them. Put it onto the bottom of the flip flop and before you start tape over it take a few strips and tape it into place so it wont move. When you think its stable enough just take some more duct tape and tape over the cardboard until it's no longer visible. Try them on and walk around. Considering they are made of tape you can always add touch ups after you are finished.
Hope you enjoy it. If you have any questions post them and i will try to answer them and add them to my tutorial.