The two wall sides of the shed with triangular peaks are the gable ends. Because the floor area is 8-foot by 8-foot, this gives you a convenient template for beginning to frame the gable end wall.
Lay one 2-by-4 board on edge along one of the sides of the floor frame. Dry-fit two other 2-by-4 boards along the two adjacent sides of the floor, making a U-shape. In the center, dry-fit four more 2-by-4 boards so that they are parallel to the two sides of the U-shape. All 2-by-4 boards should be spaced at 16 inches on-center.
Along the top side, lay two more 2-by-4 boards on-end over the top of this assembly, forming a broad obtuse angle, around 150 degrees. Use your speed square to confirm the angle. Mark the six 2-by-4 boards that run into this triangle with the pencil. Also, mark the angle on the two 2-by-4 boards that form the triangle.
With the electric miter saw cut angles on the six vertical 2-by-4 boards and on the two 2-by-4 boards that form the angle. Dry-fit all lumber again and nail into place with 3 1/2-inch nails.