How to build the base ship of your workshop?
Measure the diagonal size between the opposing corners, make sure they are the same. Keep on fiddling and shifting until they are the same. At this point you can drive in the screws or nails and fasten the rim of the base frame. Use 3 4" - 100mm nails, Although screws are quicker and less fatiguing. When your four corners are tight, check the diagonals again and your level.