Select a mold you like. It could be a flower pot, a cardboard box or any other container that's the shape and size of the pot you want to make. Line it with heavy plastic, such as a heavy-duty garbage bag.
Combine equal portions of Portland cement, sand, peat moss and coconut fibers. Double the amount of peat moss if you prefer not to use coconut fibers. Break up any clumps. Perlite and a few ounces of synthetic concrete reinforcing fibers -- added to the cement and peat moss -- are among alternative ingredients often recommended by gardening sites.
Slowly mix in water. Squeeze a handful of the mixture together; very little water should drip out if the mixture is the right consistency.
Place 2 inches of the mixture in the bottom of the mold and level it. Add more of the mixture and work it up the sides of the container, maintaining a 1-inch to 2-inch thickness as you go. Round the rim of your new pot, if desired, or add a textured design. An alternative to this method is to cover an overturned pot with the mixture instead of coating the inside. You can even cover a mound of dirt with plastic and put the mixture over it to make a free-form pot.
Insert three or four 1/2-inch to 3/4-inch dowels into the bottom of the new pot until they reach the mold, and leave them in until the pot is dry. If drainage isn't necessary, or if you'd rather drill holes later, eliminate this step.
Cover the pot and mold with plastic, and allow the pot to dry out of the sun for two weeks in an area that is at least 55 degrees. Remove the plastic and dowels, and carefully remove the mold from the pot before letting the pot sit for an additional two weeks. Use a wire brush to smooth rough corners and edges. Mist the pot with water once daily after it's uncovered to prevent cracking while it dries.
Wet or soak the pot with water at least two times daily for five days or more to leach alkali from the concrete. Alkali is harmful to plants so it's important to complete this process before putting plants in the pot. After the five days, rinse your new pot with 1/4-cup vinegar in 1 gallon of water to neutralize any remaining alkali before planting.