Insert a glue stick into the back of a hot glue gun and plug the gun's cord into an electrical outlet. Set the hot glue gun on top of a piece of newspaper and allow it to heat up.
Locate five mirror squares that are the same size and place one on top of a piece of newspaper with the reflective side facing down.
Squeeze the trigger of the hot glue gun to dispense a 1/4-inch wide bead of glue along the left edge of the mirror. Position another mirror vertically with the reflective side facing outward and press the bottom into the bead of glue. Hold the mirror in place for one minute until the glue dries.
Place a bead of glue on another side of the bottom mirror and then secure another vertical mirror to it. Repeat this process all the way around the bottom mirror until you have a cube with an open top.
Insert the tip of the glue gun into the cube and apply a 1/4-inch bead around the entire bottom perimeter of the cube and then at the junction where each mirror meets another. This creates a water proof seal.
Allow the glue to cure overnight and then fill the planter with potting soil and plants.