I picked up a pair of shorts from the thrift store, but you may already have a pair of jeans/shorts you are ready to update/don't mind if you mess them up.
Needle (optional, but helpful)
Lay jeans out and place pieces of wax paper under the area you plan on bleaching. This way the bleach doesn't bleed through the other side. Using the bleach pen, carefully draw your design. I recommend practicing with it on a piece of paper first so you get the feel for it. After, use the needle to straighten out your lines and pop any bubbles that might have formed.
Tip: Make sure your design has an even layer of bleach, otherwise certain areas will have a stronger color (in the picture above, the bleach isn't as even as it should be, I went over it again with more bleach).