Scribe a vertical line on a pre-existing rafter. Use the level to make sure the line is completely vertical and not canted off to either side. Any error you make with the line will be compounded when you take the angle for the rafter.
Place the pivot point of a rafter angle square on the bottom edge of the rafter. Place the pivot point at the exact place where the vertical line you made crosses the bottom edge of the rafter.
Align the ruler edge of the rafter angle square with the vertical line. A rafter angle square has two gradated edges. The first edge is set at a right angle to the flanged base of the rafter angle square, and is marked with a short ruler. The second edge is the hypotenuse of a right triangle and is marked with a degree scale.
Observe where the bottom edge of the rafter crosses the degree scale edge of the rafter angle square. Most rafter angle squares also have a slot cut parallel to the degree scale which displays the vertical rise in inches. This is usually marked on either side to accommodate common top cuts and hip-valley cuts. For example, a degree measurement of 30 degrees corresponds approximately to a vertical rise of seven for common cuts or a vertical rise of 10 for hip-valley cuts.
Note the degree measurement. Set your miter saw to that angle and let it rip.