Measure your existing pieces of wood to make sure you have at least two pieces of identical size for the front and back of the planter, and two other identical pieces for each side. If not, choose one smaller piece for the front and cut another large piece to the same dimensions with the saw, using the sawhorses if desired. Repeat for the sides.
Glue the edges of the sides, front and back together to create a box. Nail the pieces together, taking care not to split the wood. Place the nails about 1 to 2 inches apart for maximum resistance to wear. Use only galvanized nails for an outdoor project that will be exposed to moisture. Measure the final outer dimensions of the box. If a bottom for the planter box is desired, cut a piece of wood equal to the outer dimensions, and glue and nail this piece to the bottom of the planter. You may wish to create an open planter, however, since such planters are less prone to rot from excess moisture.
Allow the glue to dry overnight. If desired, paint the planter box, or leave it as is.