Here is a little instructable about making waterproof stencils from recycled plastic or more precisely HDPE Milk/Water Jugs. I have seen some awesome stencil "Ibles" on here like the ones using freezer paper for floating islands (Should really search the site first before I start making my own Ibles) but I feel I have a unique twist to add RE: the stencil medium and recycling factor. So let's get started!
You will need:
A Sharp blade (I used a box blade although I suggest perhaps an exacto)
Masking tape (you can use just about any kind. It's all about what you would wanna pull off your monitor screen really)
A writing Utensil (G2s are great for definition,a non bleeding sharpie would be better but a pencil would give you the most precision IMO)
A milk jug (Or any thin clearish plastic would do. We are looking for what I call "real estate". A flat area in which to cut out the stencil. I use Milk/water jugs because it's what I have and I love the idea of being able to recycle.)
A flat monitor connected to a computer (Alternately a touch screen smart phone or tablet can be used for small stencils. Also I suppose a CRT "fat back" type monitor could work as well if you find a way to use the screen as a backlight, an idea I had was to make a sturdy wooden box then a square cut large enough to sit the "fat back" end in but still hold it steady for tracing)
A cutting surface ( I used an old box)