Measure the rafters with a tape measure and mark them where you plan to make a cut. The length of the rafters will vary depending on the size of the A-frame.
Set a miter saw on a flat surface and slide the rafter for the A-frame into the saw. Push it firmly against the fence on the back of the saw. Pull the knob and rotate the blade to 45 degrees in either direction.
Line up the blade with the mark on the saw and grip the rafter firmly with your nondominant hand. Turn on the saw and pull down on the lever. Cut straight through the rafter and release the lever to turn off the saw.
Turn the blade of the saw to 45 degrees in the opposite direction and slide the second rafter into the saw. Cut the rafter just like you cut the first one. Butt the two pieces together and make sure they fit properly.