Step 1: Build the tube
Note: be sure to wrap it in A LOT OF DUCT TAPE!!!! gorrila tape works better though, and its also a good idea to take the bottom off of vegtable cans and tape to it to add more strength. a good weak point to reinforce is the bottom of the last can. it blows off really easy. i can not emphasise how important this is. if you are thinking, "this is bull, ill do what i want", no. im sorry, but ive tried it and had two launchers explode on me. it is not fun.
Step 2: Make the fill/ ignition port
be sure you have this done after you are compleatly sure that it is wrapped securely. i would reccomend using a power drill because you should have a few layers of alluminum cans on top of the pringles can. i have also found it helpfull to take apart an old pen and super glue the pen tube inside the hole to make sure that it stays clear. it is also a bonus if you can find one with a screw on end that you can plug to make a screw cap for it.