Alright with pics, like I said the camera sux but you get the idea.
1)Start with a can, can opener, any knife, and some good snips
2)Remove the lid with the can opener like so
when you are finished it will look like this
3)Next take your knife and poke a hole in the top side of the can with your knife just big enough to get the tip of your snips in(if you make it too big its ok you can fix it later...JUST BE CAREFUL THE CAN IS THIN, THE KNIFE HAS NO PROBLEM GOING IN ONE SIDE AND OUT THE OTHER)
4)Next take your snips and cut toward the bottom of the can(the can will most likely rip as you are cutting its ok you will clean that up later just go slow and rip as little as possible)
5)Finish cutting out your small square
6)Just repeat the cutting portion and get rid of the ruff edges. I personally cut several times to get the width I want you could go as big as you want now,that is up to you. Also if your light is too large it may be necessary to cut the bottom off your can to fit the light in the can.
And you are done(as you can see once I pushed too hard and the knife came out the other side, luckily my hand was there to stop it)
Hope this helps...also this is just how I did it I'm sure there are other ways