Position the planters about 36 inches apart. Measure the width of the planter sides 3 inches down from the rim. Buy planters between 18 and 24 inches high for the best results.
Cut two lengths of 2-by-2 lumber to the width of the planter side 3 inches down from the rim.
Clamp one of planks onto each of the planters, 3 inches down from the rim, checking that the planks are horizontal using the level. The side of the planters with the planks clamped on should be facing each other.
Drill four evenly spaced pilot holes, sized to fit the bolts, through the planks and through the planter sides.
Bolt the planks onto the sides of the planters, securing with nuts on the inside of the planter. Tighten with the wrench. These planks are the cleats, or supports, which will hold the slats of the bench.
Cut as many lengths of the 2-by-3 lumber as will fit across the cleats with a 1/4-inch-space between them. Cut them to 36 inches long.
Position the 2-by-3 lumber between the planters with the ends resting on the cleats. Space them 1/4 inch apart.
Secure the ends of the 2-by-3 lumber to the cleats with two deck screws through each end. The planter bench is now ready to be filled with your favorite flowers.