Select the size of the planter based on a free-form style where the planter will be directly filled with soil. Or measure the plastic tub that will be the planter liner to get the dimensions for the wood.
Style the planter so that the planks are aligned either horizontally or vertically.
Cut the planks into the appropriate lengths with a table saw or jigsaw. Taper one edge of each corner piece with a table saw.
Build a frame that will hold the pieces of one side in place. Lay end pieces in frame and place 1/8-inch separators between remaining planks. Create a wood cleat that is 3/4 inch shorter on each end. Place on each end to hold the plank together, and secure with wood screws. Repeat for all remaining panels.
Stand up all panels and hold upright using a band clamp. with thin wedges placed in front of each panel. These wedges will help hold the panels in place. Secure the sides together by screwing deck screws into the upper corners of the planter.
Drill several 1/16-inch pilot holes along each corner of the planter. Hammer the nails in, but do not go all the way through to the next panel.
Remove the band clamp, and take out the screws. Apply wood glue down each joint. Put the planter sides back together, reapply screws, and finish driving the nails through the planter sides. Finish the nails off by pushing them flush into the wood using a nail set.
Measure and cut planks to fit into the bottom of the planter. Secure with wood screws.
Fill in nail and screw holes with wood putty. Let dry and sand. Apply an exterior finish to the planter.