Foundation and Floor
Outline the shed with stakes and builder's twine. Measuring corner to corner with a tape measure and adjust the stakes until the diagonals are the same.
Set nine solid 4-inch concrete blocks for a foundation, one in each corner, one in the center of each wall and one in the center of the shed. Align blocks with a short end facing the shed front.
Lay a 12-foot long board with a 4-foot level across the tops and adjust dirt under the blocks until they are level in all directions.
Build a floor with 2-by-4-inch pressure-treated boards. Use two 12-foot boards for ends and cut boards with a circular saw 11 feet 7 inches for sides.
Nail two ends and two sides into a frame with 16d framing nails and a hammer and square it. Cut four boards 11 feet 7 inches and nail them at 24-inch intervals as floor joists between the outer bands.
Cover the floor with plywood fastened to the frame and joists with 1 1/2-inch galvanized deck screws and a screw gun.
Build three walls with top and bottom plates 12 feet long on two walls, 11 feet 5 inches on the other. Make the outer frames first with studs on each end.
Square the walls and add studs at 16-inch intervals between the ends.
Frame a door wall with top and bottom plates and end studs. Mark the center of the pre-hung door opening on the wall plates and measure half the width of the required opening on each side.
Nail a full stud on each side of the opening. Make a header with two 2-by-6-inch boards cut to that width, with a half-inch plywood spacer between boards.
Level the header at the top of the specified rough opening. Nail it in place through the side studs.
Cut four studs to fit between the header and bottom wall plate and nail them in place to support the header. Cut short studs to fit between the top of the header and the bottom of the top wall plate, with one stud on each side of the header and at 16-inch spaces in between.
Erect the walls, set them plumb and nail them to the floor frame with framing nails. Nail the corner studs together on all four walls.
Make two 12-foot long cap boards for the short sides to overlap the abutting wall tops, and two 11 feet 5 inches to fit between the other two walls.
Install prefabricated roof trusses at 24-inch intervals. Fasten truss ends to wall caps with two framing nails on one side and one on the other of each truss end.
Deck the roof with oriented strand board fastened to the trusses with galvanized nails. Cut OSB as needed to fit the roof.
Let the top panels overlap, so the edge of one panel goes over the edge of the panel on the other side. Lay roofing paper over the OSB, overlapping it at least 6 inches from the top and overlapping the peak.
Nail shingles to both sides of the roof. Nail cap shingles over the peak on both sides.
Siding and Trim
Nail 4-by-8-foot plywood panels vertically to studs with galvanized nails. Use three full panels on the sides and back.
Cut the panels to fit around the front door frame and trim them to cover the gable ends. Angle the sheets to conform to the slope of the roof decking.
Set the pre-hung double door in the rough opening and use tapered wood shims to secure it level and plumb. Test the doors to ensure they open and close properly.
Secure the door with galvanized screws through the sides and top of the door frame.
Trim the corners and edges of the shed with 1-by-4-inch boards. Nail boards vertically on corners, with edges overlapping.
Nail trim pieces horizontally below the eaves on the sides and over the seams where the panels connect on the gables. Trim the doors with vertical boards on each side and a horizontal board across the top of the door frame.
Cut a 2-by-4 to the width of the door opening and fasten it to the floor frame with galvanized screws. Set it level three-fourths of an inch below the top of the floor.
Lay a 2-by-4 alongside this ledger board out to the ground to determine a slope for the ramp. Mark that angle on the top of the board.
Cut 2-by-4's to the top angle and length to the ground. Nail them between two end boards to make a rectangle.
Set the angled end against the ledger board and secure it with 3-inch galvanized screws.
Cover the ramp with plywood from the shed floor to almost touch the ground outside. Lay shingles under the outside end of the ramp to keep the boards from touching the ground.