When you make a lampshade with real stained glass you begin with a design that will be constructed on a three-dimensional base. In this case we'll work the other way around. We will start with a three dimensional shape (PET bottle) to which the design sill be adapted.
You can use a whole lamp shade design to adapt to your PET bottle or you can use just the interesting bits of the design and hand-draw the rest, that's what I did.
Here's the URL of the dragonfly design I used, but there are a lot more designs to choose from in other websites with free stained glass designs.
WARNING: This lamp uses electricity and there is a risk of electric shock. If you have never worked with electricity before, do not attempt to make the lamp by yourself. But you still can make the lamp shade.