Mix the concrete. In a separate container, mix the concrete with water until the consistency of a thick oatmeal. The more water you add, the smoother the consistency, but your project will also take longer to dry.
Prep the Molds. Wipe the inner and outer molds with motor oil or release spray. Make sure to get crevices or corners well. Place the molds on a level surface.
Fill the mold. Fill the bottom of the outer planter to the depth of the bottom of the inner mold. Place your mold inside and fill around the sides evenly with concrete mix. As you work up, shake the container to get rid of bubbles. Use a board or heavy item to keep the inner mold in place.
Remove the Mold. Once the concrete is set up - i.e. very hard but not dry, you can release from the mold. The longer you wait to dry, the harder it will be to release, but not too early because edges may stick to the mold or become brittle. For the largest pots, we waited about 12 hours. Allow to fully cure and dry.
Seal and decorate. Concrete is very porous and sealing is required to make your pots last. Since these planters will be used outdoors, I sealed them with a clear acrylic sealer like this one, inside and out.
How To Make Self Watering Planters
Cut a plastic bottle in half - 2 liter soda bottles work great for large planters.
For large planters, place an old clean t-shirt through the cap so it can reach the bottom of the planter. For smaller planters, poke a hole through the bottle cap and thread a long string through the hole. The string should be made of cotton or other wicking material. I find the easiest way to put a hole in the lid is to warm the plastic with a lighter and then punch a hole. The string should not be pinched and there should be several inches coming from the top.
Place it cut side down in the bottom of the planter. The top of the bottle should not be higher than half the depth.
Drill a small overflow hole into the side of the pot. The hole should be at least 1-2 inches below the top of the lid.
Place some gravel around the soda bottle. Lay the potting mix over top of that and plant as desired.