[header] How to make concrete planters [title] 1-choose your molds. [step] You will need an outer mold and an inner mold of same shape but different size. [title] - the smaller mold should nestle inside the larger with a gap of about 2 inches or 5 cm. 
[step] The gap between molds will be the thickness of your planter's walls. If your planter will be any larger than 2 feet (0.6 m) by 2 feet (0.6 m), the walls should be 3 inches or 7.5 cm thick.