If you've never made anything out of papercraft before, I suggest you look elsewhere Papercraft Museum and make something out of paper, it takes a significant amount of more effort (and time) to work with aluminium.
Step 1: Flattening Aluminium Cans
The method for flattening the cans that I've found works best is to use a sandwich maker (an old one, don't ruin your good sandwich maker!!). Before turning it on, lay down a piece of baking paper on the bottom, put about 4 cans in, then add another layer of baking paper on the top (this should help prevent at least a little bit of damage to the sandwich maker).
Hold the cans down flat as you close the lid, once you've done that, turn the sandwich maker on.
I haven't fully experimented with getting the best efficiency here, but I left it on for 10 minutes with 4 cans in there, then turned it off and let it cool before I opened it.
Taadaa! Flat cans!
Other methods I have tried are:
* Ironing (Does nothing)
* Ironing and bending at the same time. This method works, but you end up with your piece looking a little bit bent as it's hard to get it as flat as you would in a sandwich maker.
* Clamping it between 2 pieces of wood. This does absolutely nothing, if you leave it for a day or a week, they bend right back to their natural shape.
It seems like heat is very important in getting these things to stay flat.