With all of the biscuit slots cut, we'll assemble the plywood planter box. Begin with the two ends and the bottom. Place a small amount of glue in the bottom slots of the ends and insert the appropriate #10 or #20 biscuits (depending on what size slots you cut in the previous step). Place a bit of glue in the corresponding slots of the bottom and attach the ends to the bottom. Tip the unit on its side and drive some finish nails (preferably using a pneumatic finish nailer) through the plywood bottom into the end pieces. Attach both end pieces to the bottom in this manner.
Next, place glue and biscuits into the side piece of plywood that will fit onto the edge of the assembly facing upward. Then, place glue in the slots of the partially assembled box and fit the side onto the assembly, aligning all of the biscuits with the slots. Tap the side piece of plywood into place as shown in the picture above.
Adjust the alignment appropriately and tack the side into place with finish nails. Then, repeat the entire procedure on the opposite side.