If you have either of these air tools, you could use them for a fast, easy, and very clean cut:
If you do not have access to these tools, then I would recommend using something like this:
You may also want to use a drill to get some starter holes on the vertical cuts. Or you could cut both top and bottom, then bend the side inward (not as clean, but works and not visible after installation).
Whatever cutting tool you choose to use make sure the blade is for cutting thin steel. Using the wrong blade could make it harder to cut and the cut would not be as clean, possibly nasty.