Summarize:

Measure the length of the section of wall you're covering with siding. Then, subtract that amount from the length of the piece of siding you're going to cut. The total you get is how much you'll need to cut off the siding. For example, if the section of wall you're covering is 5 feet (1.5m) long, and the piece of siding is 9.5 feet (2.9m) long, you'll need to cut off 4.5 feet (1.4m) from the siding so it fits on the wall. Use a carpenter's square to draw the line straight. Make the line as dark as possible so it's visible when you're cutting it. If you're cutting dark vinyl siding, use chalk or light-colored tape to mark the line you want to cut. Grip it near the line you marked so the siding doesn't shift as you're cutting it. As you're cutting the siding, avoid closing the blades on the tin snips all the way so you get a cleaner cut. Keep cutting until you get all the way across the vinyl siding. Wear safety goggles while you're cutting the siding. Use the piece you just cut to see where you need to mark the lines on the other pieces. Lay it on top of another piece of siding and trace the cut edge onto the other piece.
Use measuring tape to measure how much siding you need to cut off. Mark the line you want to cut on the siding with a pencil. Hold the siding steady with your non-dominant hand. Cut along the marked line using tin snips. Repeat on the rest of the vinyl siding.