I had some Gila window film left-overs, and a few hours waiting around for a guy to come over to buy some old junk. I got it in my head to make stickers from the film. (Yeah, I know: "what a dorkazoid.") Here's how I did it. I'm assuming you have film installation supplies:

1. Get a design. Cut a piece of film to fit it. Figure out which side has the clear backing, and which side has the window film. Put the film on the design, backing side up (window film side down). (To figure out which side is which, peel it apart using a couple bits of tape.)
2. Using a permanent marker, trace the image onto the backing. This is necessary because the film will slide around as you cut it.
3. Using a sharp knife, cut the design. I used a square of vinyl tile as my cutting mat. It's important to use something like that so you can dig the knife through the film.
4. Clean off the window. Wet it down with water, and scrape it with a new razor. Wet it down with window cleaner, wipe, and then wet it with water and clean that off.
5. Take a large plate and put some water into it. Spritz it with a little of the regular film wetting solution (10 drops of baby shampoo in a pint of water, in a sprayer). Peel the backing from the sticker and slide it into the water. This helps keep the sticker flat, and softens the adhesive.
6. Spray the window down with the wetting solution until it drips. Put the sticker on the window, flattening it down, and sliding it into position.
7. Spray the back with wetting solution, and then use a squeegee to work out the bubbles. (This is important, because it flattens down the surface so you get a nice, even reflection.) Be gentle with the squeegeeing, because you might cut or bend the sticker.
8. Let it dry out for a while, then it's done.

I found that it's easier to make big, bold designs than intricate, spindly designs. The spindly ones tend to stick to themselves.

Here's what you need: cutting mat, blade, some film, and a pen.

Not pictured: slippery spray.

Here's my second sticker ever:

Here's my car sticker. I messed up and drew the image onto the window film side. This left some marks on the sticker. It's no big deal; this'll wear off soon enough, or it'll get scraped off.