Step 1: Getchyer Materials
First is-duh-get the supplies you'll need.
Heat shrink tubing
Piezo igniter from an old fireplace lighter
Pointy pokey thing
Something that will heat-shrink the heat-shrink
A side note: there is another Instructable now by Geoffrito about how to remove the ignitor. You don't need to add extensions unless you want to, just extracting the piezo igniter should give you enough length.
Step 2: Puncture the Lid
Film canister lids typically have an indention in the exact center. Most people think that they are part of the molding process or something, but those of us in the know realize that their true purpose is to help us geeks who want to poke holes in them.
I drilled a small pilot hole, then carved out the rest with an X-acto knife. You could also drill the whole thing or stab it with a hot nail, I don't really care. It's your film canister.
Step 3: Add Wiring
Now, poke the wires from the piezo igniter into the hole. Make sure that the bare ends go on the inside, then put a bit of heatshrink tubing or tape to hold it. Then, practice your spark plug-making skills and bend the wires so that there is a small gap. When you click the igniter, there should be a small blue spark between the bare ends.
Step 4: Shoot Things
To fire the cannon, squirt some fuel in, seal the cap, and click the igniter. With a nice bang and a lovely yellow fireball, the can part will go shooting across the room. This cannon is low-powered enough that it's perfectly safe to fire indoors. It's really fun to fire it next to a sleeping cat.
A quick note on fuel: Experiment! I used aerosol hairspray, which is very flammable and comes out in a nice mist. That mist part is the most important. If you just dribble in fuel, it probably won't fire.