Once you have cut each side to the proper length, it is time to cut the miters at each end of the shelf sides. Set the saw to cut 30-degree miters.
Now that the miter is set, line up the corner of the shelf side furthest from you with the blade edge. The miter will cut the end of the board at an angle. Cut each end of the six side pieces this way, for a total of 12 cuts.