Trimming sheetmetal properly requires some tools, and you probably have at least a few of them in the garage. You will need some kind of cutting device, a hammer, pliers, and a grinder of sorts. We’ve used everything from tin snips to grinders to cut sheetmetal. You can use a saber saw, jigsaw with a metal cutting blade, die grinder, rotary tool, or angle grinder with a cut-off wheel (although cutting radiuses is nearly impossible with these), tin snips, hole saws for lights, and so on. The best tool we have found so far is a high-quality air saw. Also, taping off where you are going to cut can help protect the paint near the cut.