Lay out a rectangle 18 by 10 feet using a measuring tape. Mark each corner by lightly tapping rebar into the ground with the sledge. Square corners by measuring and equalizing opposite diagonal distances. Drive corner rebars straight in 15 inches.
Stretch twine between corners, to make a rectangle for guiding rebar positions. Drive rebar into the ground every 36 inches between corners on the sides.
Clean 2 inches of every pipe tip and the inside of every tee and cross connector. Slide a 10-foot pipe over every rebar. Have someone assist you to bring together the tips of opposite-side pipe ribs, starting at the end. Coat 2 inches of pipe tips and inside of a tee connector. Quickly slide pipe ends into opposite connector ends and point the remaining opening toward the other end of the shed, level with the ground. Repeat at the other end.
Keep cross-connectors parallel to the ground as you glue and slide in remaining ribs. Use the same technique to connect tee and cross connectors with 34-inch pipes. Allow glue to set and dry.
Stretch an 8 feet by 12 feet tarp at the ground between two end corners of the rib structure, leaving 9 inches of tarp on the ground. Wrap around the pipe and clip in place with binder clips. Sink tent pegs through the grommets on the ground. Raise the other tarp edge over the top of the pipe arch, stretch tight and fasten with binder clips on either side of the tee connector. Use clips to wrap and fasten remainder of the tarp in place every 2 feet.
Repeat Step 5 at the other end of the shed, but leave about 6 feet of one corner unfastened as a door. Lay the 20 feet by 20 feet tarp over the rib structure. Stretch and fasten the ends through the grommets with bungee cords and tent pegs. Stretch and fasten side grommets down over rib footings with ball bungees.