Place the bevel tool's handle flush to the corner of the wall at the point where the roof angle begins. Push the tool's arm up until it's flush against the roof eave. Check that both the handle and the arm are flush against the structure, and tighten the wing nut to prevent any movement.
Set the bevel tool on top of the vinyl siding. Flush the bevel tool's handle to the butt end of the siding with the angled arm resting over the top face.
Trace along the angled arm where you'll make the angle cut on the siding. Line the edge of a straightedge tool, such as a 4-foot level, up to the traced line if the layout line needs to continue past the bevel tool arm length. Trace along the straightedge tool to create the cut line.
Install a fine-tooth blade backwards in a circular saw. Adjust the circular saw's cutting depth so that it's slightly deeper than the thickness of the siding. Rest the front of the circular saw table on top of the siding and line up the blade to the layout line. Start the saw by squeezing the power trigger.
Allow the blade to rotate at its full power before pushing it slowly into the vinyl siding. Follow the line across the siding with the saw. Pull the saw away from the siding once the cut is complete, and dry-fit the vinyl siding before installing it.