Summarize:

Cut 3 2 by 8 in (5.1 by 20.3 cm) boards to 16 feet (4.9 m). These boards will form part of the mudslips or bottom of the shed. Also, trim 2 2 by 6 in (5.1 by 15.2 cm) boards to 16 feet (4.9 m), which will form the other part of the mudslips. Finally, cut 15 2 by 6 in (5.1 by 15.2 cm) boards to fit across the center of the floor, going from one mudslip to the other. They should be 10 feet (3.0 m) each. Set these in one area so you know they're for the foundation. Cut 26 boards to the height you want at the back of the shed, typically 6 feet (1.8 m); this equals 6 boards for the back wall and 10 for each of the sides. For the front, cut 4 boards that are 12 feet (3.7 m) high.  Also, cut the boards for the tops and bottoms of the wall frames. Start with 4 boards that are 113 inches (290 cm) long for the front and back walls. For the sides, cut 4 boards that are 192 inches (490 cm) long. Always measure carefully before cutting the boards. Draw out the schematic for the triangle on a piece of plywood. The triangle will need to be 188.5 inches (479 cm) on the bottom. Measure a right angle, and then create a side going up that's 72 inches (180 cm) tall. Draw a line from the top of this edge down to the other end of the board on the bottom, forming a triangle. Cut pieces to fit in the triangle.  You'll need to cut the hypotenuse (the side that doesn't touch the right angle of the triangle) at an angle on both ends to meet the other boards. Cut and measure 4 boards to go vertically in the triangle. Each board will be a different height, and you'll need to cut the top of the board at an angle. Cut enough boards for 2 triangles, one for each side. Cut 2 2 by 8 in (5.1 by 20.3 cm) boards to 10 feet (3.0 m), one each to go on the top of the walls at the front and back. Cut 11 2 by 8 in (5.1 by 20.3 cm) rafters for the roof. The rafters will run from the top of the front wall to the top of the back wall. Measure how long you want yours to be when you get the roof up. They will need to be at least 19 feet (5.8 m) to make an overhang of 1 foot (0.30 m) on each end.
Trim the foundation boards to size. Cut 2 by 4 in (5.1 by 10.2 cm) pieces to length for the walls. Make the pieces for the triangular roof braces. Trim the boards for the roof.