I set up a 6m x 6m square wood frame, by using 2m wood beams.
Within the frame, plant 5 x 5 seeds (total 25 plants).

First lower the ground level so the 2m wood beams are sunken but still visible, once you raise the ground level back up. The goal is to be able to walk over it without clipping.

The Harvester tool creates a 3m x 3m section of fertilized land, so it fits 4 "Harvester Clicks" of fertilized land inside quite nicely.