Measure with a piece of string the circumference of your head; this is where the bottom of the hat will lay, so consider where you want your hat to rest, also don't forget about your ears. I always have hats that are too small to include my pony tail so I was sure to include that in my calculation.
Now cut a piece of duck tape of that length and fold it in half the long way so that the sticky sides come together. Make a circle with that piece of tape and then tape it together.
Now measure with a piece of string from the front of your forehead and the back of your head. Imagine this is going to form the top of your hat so you will not want to make it too small or large. Again cut a piece of duck tape to this length and then fold it in half the long way so that the sticky side will be on the inside. Now cut another piece this same length and do the same thing to it as you did to the pieces before. Make an X with those two pieces and tape them together. Then take the ends of the two pieces used to make an X and tape the ends to the circle that you made in the start.