I do realize that there are numerous other instructables just like this one, but I have been doing this for years and I figured I would post one myself. All you need is a pair of scissors, an old giftcard, a pen, and a file. You can also use a real guitar pick for a template.
Step 1: Tracing
Take the guitar pick and trace around it on the giftcard. Try to do it as close to the edge as possible so you can get multiple picks out of one card. Then, cut out the tracing using the scissors. If you don't want the pick to be the normal shape, then cut out your own unique shape.
Step 2: File It Down
Then, file down the edges of the plastic. After this, you can use it on your guitar.