What's the best way to cut metal siding?

The method you use depends on if you want the steel to have tiny rust spots all along the cut area or not. I work for the largest pole barn type steel maker in the U.S. We strictly tell installer never to use a circular saw, metal cutting wheel, or any other power saw to cut steel. The reason is that the tiny shards of steel from the cutting are so hot they will burn right through the paint and the zinc coating under the paint. Within a year you'll have rust spots all along the cut. The only approved way to cut steel is with a hand snips, a power snips, or a nibbler. The power snips is the easiest method and the fastest.