Prepare the shed location by raking off all debris and leaves. Level it as much as possible and make sure the ground is firm and there are no drainage problems. Avoid any underground utilities or tree roots. Mark the basic outline of the shed with stakes and builder's twine. Measure corner to corner with a tape measure to square the outline; adjust the stakes until the diagonals are equal.
Make a 5-by-5-foot floor frame. Use two 5-foot 2-by-4-inch pressure-treated boards and two cut with a circular saw to 4 feet 9 inches. Nail those into a square with a hammer and framing nails; check the corners with a framing square. Add a center beam--also 4 feet 9 inches--in the middle of the frame; place it in the center between what will be front and back.
Set the frame inside the outline and place 4-inch solid concrete blocks under each corner. Add two blocks under the center beam for extra support. Put a long board with a level on it across the frame and adjust the dirt under the blocks until the frame is level in all directions. Cover the frame with 3/4-inch pressure-treated plywood nailed to the 2-by-4s.
Build four walls with 2-by-4 top and bottom plates and studs. Frame the outlines first, with plates and corner studs. Use 5-foot plates on two walls and 4-foot 5-inch plates on the other two walls to accommodate the width of the other walls. Set interior studs in three walls 20 inches in from each corner; normal wall studs are placed at 16 or 24-inch intervals but those spacings don't work with 5-foot walls.
Frame the fourth wall for a door. Add a second corner stud on one side; insert two 6-inch long 2-by-4 spacer blocks between the corner stud and the double, nailed in place through both studs. Put another stud the distance of the rough opening for a pre-hung door, probably about three feet. Nail a 2-by-6-inch header board between those studs at the height for the door frame. Put two more studs, cut to fit, between the bottom of the header and the bottom plate. Use a reciprocal saw to cut off the bottom plate inside those short studs. Add at least one short stud between the top of the header and the top plate and another full stud centered between the door outline and the other corner stud.
Erect the walls, set them plumb vertically and nail them to the floor frame, and nail corner studs together. Tie the walls together with cap boards. Put two 5-foot caps on the end walls to overlap the side walls, then cut two caps 4 feet 5 inches for the other two walls. Add a second cap board on top of the door wall to provide slope for a roof.
Lay a 2-by-4 flat, 4-inch face up, alongside one wall cap and mark the angles of the caps at the end, to cut it to fit between the two end caps. Cut three 2-by-4s like that and nail them between the end walls to make rafters for roofing. Brace the rafters with metal angle braces on each end; these fasten to both the rafter and the cap board to hold them together.
Cut four 1-by-4-inch boards 5 feet long and nail them across the rafters, one at each end and two 24 inches in from each end. Cover the roof with corrugated vinyl panels. These typically come in 6-foot sections with sides that overlap to cover 24-inch intervals. Use three panels. Let the ends overhang the walls by 6 inches on each end. Fasten the panels to the 1-by-4s with galvanized screws with plastic washer caps, with edges overlapping 6 inches on each side. Put screws in the corrugation valleys and overlap seams by one peak and one valley.
Use board and batten cladding for the walls. Cut 1-by-12 planks 5 feet long and nail them to studs horizontally. Trim four planks lengthwise to fit between the top horizontal plank and the roof junction. Seal the corners with 1-by-4-inch trim boards with edges overlapped. Add 1-by-4 trim at the wall tops to cover the space under the roof line. Finish the horizontal joints with 1-by-2-inch batten boards nailed to the planks.
Install a pre-hung door. Set the frame in the rough opening. Use tapered wood shims to square it and level it; check all sides with a level to make sure they are plumb and level. Fasten the door frame to the rough frame with long screws driven through the frame into the studs with a screw gun. Paint if desired or leave cedar siding unfinished but sealed with a clear protecting coat.