Measure the length of pipe you need to cut with your tape measure and mark your piece of pipe accordingly. Use the angle finder to determine the angle you need to cut the pipe at to wrap your corner. Unfold the tool and place it where the pipe will eventually rest and adjust the two arms until you have your angle. Transfer that angle to your pipe and draw it with the grease pencil.
Move the piece of pipe to the tray of your miter saw. Place the piece of pipe on the tray and ensure that it is securely placed up against the back side of the tray and stable across its length. Adjust the angle of miter saw depending on the angle of your cut by loosening the swivel bolt on the saw, moving it to the angle you need and then tightening the bolt for cutting.
Stabilize the piece of pipe with your off hand and pull the trigger of the miter saw with your main hand. Press the blade slowly down into the piece of pipe and cut it through according to the mark you created earlier. Let the blade cut naturally rather than forcing it through, as this will cause the blade to bind up and stall.