Measure the length of board that is needed. If you're installing the board along a wall, measure from corner to corner.
Measure the angle needed with the protractor. This is the combined angular distance needed by the joint. Divide this angle by two and note the result.
Mark the length needed on the board with the pencil.
Set the board on the work surface in the same position as if it were installed.
Rotate the deck of the miter saw to the angle previously calculated.
Set the lumber on the deck of the miter saw and adjust the board directly under the cut line. Clamp the board to the back fence, if needed.
Bring the blade down quickly to make a precise and clean cut.