Especially useful for reloading Instamatics, since the film is getting hard to find. See my Pocket Instamatic Instructable.
Just get a few used one-time-use cameras free at any photo lab.
I prefer the ones without a flash, but I take what I can get.
The shock you get from the flash unit is molar-melting, so play it safe and get the outdoor, sunny-day, no-flash type cameras if you can.
You'll also need a pack of snap-off cutters from the dollar store, some 5 minute epoxy, a metal nut, and masking tape.
Step 1: Gut the Camera
Peel off the paper cover.
Next, remove the front by gently prying up the latches on the sides.
Discard the flash unit and be prepared for you WILL get shocked.
Remove the two plastic flaps on the camera bottom, the inside mechanisms and optics, and back cover.
We just want the camera as a film holder.
Step 2: Blade Holder
Buy a 6-pack of snap-off cutters from the dollar store.
Snap off two blades using the knife cap.
Use a metal nut about 15mm across to make a blade spacer.
Epoxy the knife blades to either side of the metal nut. The long side of the blade should line up with the side of the metal nut. The triangular cutting tip should protrude above the top edge of the metal nut.
The epoxy sets hard enough for handling in about 5 minutes. It takes 8 or more hours to completely cure.
Mask off the edges of the camera film path with regular masking tape to protect them from the globs of epoxy you will use to set the blades:
Now epoxy the metal nut with the two blades into the film path with just the tips sticking into the film path. The cutting edge should slope towards the loose coil film holder side of the camera.
Step 3: Completed Slitter
You can see the silver metal nut stuck in the glob of black epoxy.
The two tips just stick into the back of the camera when you press it in place.
The plastic key is a piece from a garden sign. It has a X cross section and works great for winding the film.
You can also use a car key.
If the blades ever wear out, or become dull, or out of alignment, you can usually pry the entire lump of epoxy out of the camera and start over.
Step 4: Using the slitter
Loop the narrow film leader back and tape it down.
This keeps the film from rewinding all the way back into the 135 cartridge.
In a dark room or bag, coil the film as you unroll it from the metal cartridge.
Then load the coil and the cartridge into the film slitter.
You can see more information at my other Instructable - Pocket Instamatic.