- 1). Cut 3/4-inch plywood to form the sides and shape of the mould. If you are making a concrete planter, you will need to make two moulds--one for the outside and one for the inside. Make sure that the moulds will allow the concrete to be at least 2 inches thick.
- 2). Drill holes in the plywood where the pieces will join, using a drill bit that is 1/16 inch smaller than the diameter of your wood screws. The holes should be no more than 4 inches apart so the moulds can withstand the weight of the concrete. Attach the plywood pieces with wood screws, using the drilled holes as guides.
- 3). Coat all plywood surfaces that will come in contact with concrete with release oil. This will allow you to remove the moulds from the concrete once it has cured.
- 4). Mix the concrete according to package instructions. Use ready-mix dry concrete to save time and make the project easier.
- 5). Pour the concrete into the plywood mould. If you are making a planter or other container, fill the bottom of the outer mould with 2 inches of concrete, then center the inner mould on top of this layer. Fill the space between the outer and inner moulds with concrete mix, using a tamper to compress the concrete as you pour. Smooth the top of the concrete with a trowel.
- 6). Allow the concrete to cure. In most areas, concrete will cure in 24 hours; if you live in a particularly humid climate, allow it to set for 48 hours.
- 7). Remove the wood screws and pull the plywood mould from the concrete. Clean the mould surfaces thoroughly so you can reuse them. Let the concrete continue to cure for five days before transferring the decoration to your garden, patio or other outdoor space.
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