Very carefully, these cuts can be mighty sharp before some sanding/filing/bending
First, easiest and fastest choice would be an angle grinder.
A hacksaw will take longer, but will work too. Keep the saw at a shallow angle and use both hands to guide it.
To have the hands free for grinder/saw fix the can with a vice or clamp, either top to bottom or with a piece of wood (or whatever) in it that keeps it from being flattened or slipping out of the vice, a board cut to fit just inside works great.
For the saw a second person fixing the can work too - sturdy working gloves for holding the can always recommended, also for sanding the edges :?
For the grinder (or most power tools): Personally, I do really not like any other hands close I have to watch out for.
Sheet metal scissors are an option too, will require some rebending work, and it takes some practise to get a straight cut (punch hole with a knive or so to start the horizontal cut). Also normal scissors will do, sturdy ones, but that'll dull them so be sure they aren't your moms :evil: favorits, they'll be pretty much ruined for anything else.
Also a sturdy, sharp knive can work, starting at a hole in the corner and using a small hammer to drive it carefully, but again: :evil: . Use cutting pliers for the rim.
And have band-aid handy, just as a superstition to trick Murphy - you'll most likely need them if there aren't any