Stuck on building a greenhouse

what sort - a polycarbonate kit or something from scratch? if it's a kit, the usual place it goes wrong is the base. it has to be dead square and dead level before a single pane goes in or nothing lines up and the glazing clips won't bite. measure the two diagonals, they should match. build the whole frame, square it, then glaze last on a still day. if you're halfway and the frame's already racked, loosen every bolt, pull it square on the diagonals, then tighten.