Mark the cut location for the rafter based on the pitch of the roof. The pitch is the rise of the roof over the run. Most roofs are approximately 7/12 pitch, meaning 7 inches of rise for every 12 inches of run.
Set a rafter board on a flat surface and lay a framing square on it. Hold the tongue side of the square with the smaller number in your pitch on the inside edge of the board. The tongue side is the shorter and thinner side of the framing square.
Angle the framing square to align the larger number to the outside edge of the framing board. The tongue side will align with the number 7 and the body or longer side of the square will align with the number 12 on the square. Draw a line that intersects the two numbers for the plumb cut.
Lay the rafter board on a flat surface. Line up the mark with the blade of a circular saw. Turn on the circular saw and run the blade of the saw straight along the mark until the board is cut. Cut along the second mark on the rafter board with the circular saw.
Butt the two pieces together and verify they fit properly and the correct angle cuts were made.