The first thing you will need is a safety can opener to remove the top of the can, which will become the lid. I picked up this Farberware model at the local Walmart store for about ten bucks.
Since sides of the Heineken can angle out from the top, the opener needs to be modified to accommodate. For this I used my Dremel tool, but you could also use a hack saw or something similar if you don't have a Dremel. Cut off the bottom edge of the opener at an angle so it will fit squarely on the edge of the can.
Then use some sand paper to smooth out the rough edge you just made.
Now the can opener is ready to use. Simply run it around the perimeter of the can while turning the knob. Finally, sand the top of the can and edges of the lid to remove any sharp edges.
My can didn't come out nearly as clean as the one I received with my Trail Designs Caldera Keg-H, but I'm betting they use something more sophisticated than a Walmart can opener. MYOG stuff doesn't need to be perfect any way, and this one works just as well. With some practice, I think I can to get better at removing the tops cleanly, but then I will have to drink more beer, which will lead to more hiking to burn the calories. It's a vicious cycle!