Step 1: Materials
A sewing machine. A sewing machine is not essential but it will make your life a good deal easier. It doesn't have to be anything fancy and you could probably get one at your local used store for less than $30. Another option would be to borrow it from a friend. In either case chances are the manual is long gone and you won't know how to thread it. A quick Google search of the serial number will help you enormously.
About two yards of cloth. I used a fleece material that I had left over from my Deadmau5 head so it was like a pair of yoga or sweatpants. I recommend flannel but really anything will do.
A spool of thread to match the color of your cloth.
A pair of pants that fit you well.
A very large piece of paper. You should be able to wrap it around yourself at the waist and it should reach the floor.
Elastic. If you don't want to buy it you can scavenge it from an old pair of shorts, pants or underwear.