For the long sides: Take two planks and lay them down on the sidewalk on top of and perpendicular to three or four short lengths of 2x2 or 2x4. Make sure everything is square and aligned. The planks should be slid down to one end of the support beams. Screw it all together. Repeat until you have two long side pieces.
For the short sides: Have someone hold the two long sides up as they would be in the final position for installation. Take two short lengths of planking and fasten them with screws to each end. When you are done you should have something that resembles my photo.
I chose to have the support beams extend upward to allow the use of string and fabric to protect and support my plants as they mature through various phases of their growing cycle.