First I cut the piece of foam to fit and put it in place. This is not needed in most setups but I like it here. Notice that it is now notched to fit around the 2x2 supports.
I needed more room for the larger plants so the bottom lights went on the center, 24" high, supports. Some 52" scrap boards make a shelf on the 32" high supports for a shelf and the final set of lights goes on the top to make a place for smaller plants. It is a good idea to have the lights as close to the plants as reasonable.