Summarize the following:
The boxes will be the molds that you’ll use to create the planters. Find 1 large box and a slightly smaller box. The smaller box should be able to fit inside of the larger box. The space in between the 2 boxes will determine the thickness of your concrete planters.  The boxes can be made of cardboard or wood. The smaller box should be at least 2 inches (5.1 cm) smaller than the big box on each side. The smaller box should be at least 8 inches (20 cm) deep for larger plants and 3–4 inches (7.6–10.2 cm) deep for smaller plants. Get sturdy boxes so that the mold doesn’t break when you fill it with concrete. If you want a concrete planter that’s a different shape than a rectangle or square, you can purchase plastic containers of any shape to serve as your molds. Find 2 different sized plastic containers online or at a department store.  The smaller container should be at least 2 inches (5.1 cm) smaller than the larger container around each side. The smaller container should be at least 8 inches (20 cm) deep for larger plants and 3–4 inches (7.6–10.2 cm) deep for smaller plants. For instance, you can get 2 pots or 2 plastic bowls. Stretch the trash bag over the smaller container so that it's taut against the sides. The trash bag should be tight against the inside of the container as well. Tape the trash bag to the box or container with duct tape. The plastic bag will prevent the concrete from sticking to the small container and will make it easier to remove the mold. Use a ruler to measure 2 inches (5.1 cm) from the base on the inside of the large container. Make a horizontal line with a pen or marker. This will help you determine how much concrete you should initially pour into the mold.  If you want the sides of your planter to be thicker, measure and mark the line to reflect the change in dimensions. For instance, if your planter's walls are 4 in (10 cm)-thick, you'd measure and mark 4 inches (10 cm) from the base of the planter. Spread the non-stick spray around to get an even coating on the inside of the larger box. The cooking spray will make it so that the concrete doesn’t stick to the sides of the larger container.  You can purchase a can of cooking spray online or at a grocery store. You can coat the inside of the larger box with mineral spirits as an alternative to cooking spray. Pour the spirits into the bottom of the box and spread it around on the inside of the box with a rag.

Summary:
Get 2 different sized boxes. Use plastic containers rather than boxes. Tape a plastic trash bag around the smaller container. Mark 2 inches (5.1 cm) from the bottom of the large container. Coat the inside of the larger box with cooking spray.