After measuring your elastic, tie a knot on one end (making sure to leave some space to tie it when you're finished), thread a bead, and tie another knot to keep it in place. Then leave a gap about the same length of one of the beads, tie another knot, thread another bead etc...
Do this until you are in the middle of the plastic.
Cut a little pair of scissors out of shrinky dink (mine was around 3-4cm long and came out around 1cm long) and make two very small holes in the middle - this is where you will thread it later. Colour it in with permanent marker and bake it in the oven! (baking instructions are on the packet) You may also want to go over the edges in marker when it comes out of the oven.
Thread the elastic up through one of the small holes you made and down through the other and slide it down to the middle of your bracelet.
Carry on knotting and threading beads until it is the required length (make it tight!)
Tie the two ends together very tight! You want it to look tiny so that when you put it on it stays straight and doesn't bunch up!
There you go! Just don't literally cut off your wrist...