Lay two pieces of lumber upright and perpendicularly with ends touching to form a 90-degree angle. Drill three 2 1/2-inch evenly spaced holes through the overlapping piece of lumber and into the adjoining piece of lumber.
Drill 3-inch screws into the three holes.
Repeat Steps 1 and 2 with the remaining two pieces of lumber. Join the four pieces together to form a square.
Drill 2 1/2-inch holes into the overlapping pieces of lumber and into the adjoining pieces. Drill 3-inch screws into the holes.
Measure and cut landscaping fabric 3 inches larger in length and width than the inside dimensions of the planter.
Lay the landscaping fabric inside the planter so that the ends fold 1 1/2 inches up the inside walls. Staple the ends of the landscaping fabric to the planter walls with a staple gun.
Set the planter in the desired position and fill with potting soil.