The Walgreens forever camera is a disposable camera that you reuse over and over again. After using up the film they include in the camera, you return the camera and get the film processed. They load a new roll of film in the camera for free. The deal is that as long as you bring the camera back to them for processing they'll put a new free roll inside. The trick is that you can take the film out and put it in a different camera without much trouble. This allows you to pay $2.15 + tax for film processing and a roll of 27 shot color 400 film.
Step 1: Wind the film back in the cartridge
When you get the camera it will already be loaded with film. The camera works opposite of how a normal 35mm film camera works. With a 35mm camera you put the film in the canister holder then take take pictures and pull the film out of the canister to get to the next frame. When you're done you wind the film back into the canister and take it out. With the Forever Camera and many other disposable cameras the film in inserted, then wound out of the canister. when you take pictures you're winding the film back into the canister.
If you were to open the film door you'd expose the roll. You need to "take" pictures and just obstruct the lens and make sure no light gets in. After going through all 27 frames you'll get to the E in the frame counter on top of the camera. Now the film is wound into the cartridge and ready to be removed.
Step 2: Put film in another camera
After removing the film, put it in your favorite 35mm camera or whatever other photographic creation you have and shoot away. You should get around 27 shots.