Build the preliminary skeleton of your sculpture out of scrap wood and cover it with chicken wire. You can attach the wire with staples, or simply tuck it into joints in the wooden structure. This only need be the general shape of the final sculpture, as the cement will fill in the body and create the final skin.
Mix a batch of concrete in a 5-gallon bucket. Follow the directions on your particular bag for the water-to-cement mix ratio.
Fill the major gaps in the sculpture skeleton with cement using a trowel. Keep stirring the cement between applications, so it doesn't change consistency. Fill in the body before moving to the detailing stage.
Apply the details to your sculpture. For a smoother finish, wet down your gloves in water to thin out the top layer of the cement and smooth it. Let the finished sculpture sit for 48 hours, then apply sealant.
Apply the sealant with a paint brush. The sealant both waterproofs the statue and leaves it with a slightly glossy shine. The sealant dries in one to two days. If you wish to paint the sculpture, do so before applying the sealant.
Nail several 2-by-4s into geometric shapes (rectangle, triangle, square) and cut plywood backing boards for them. This is the basis for a geometric abstract concrete sculpture. It doesn't matter how many shapes you have; make as few or as many as you want in the final sculpture.
Mix concrete in 5-gallon buckets. Pour the mixture into your open geometric box shapes. Let the shapes cure for 48 hours.
Remove the concrete from the wooden frames by pulling out the framing nails or using a chisel and hammer to break the nails between the 2-by-4s.
Arrange the concrete pieces by lying them against each other or lining them up in a row.
Paint or stain each shape a different color. Primary colors work great for three-shape designs. Red, yellow, green and blue work best for four-shape designs, but the color palette is completely up to you.
Seal the concrete after the paint or stain is dry.
Purchase a pre-made concrete sculpture form online or from a garden statuary supplier. These are usually one-time use forms, but can be bought in bulk if you want several of the same sculpture.
Mix the concrete in a 5-gallon bucket.
Pour the concrete into the form. For most forms, a built-in funnel is at the top; if there isn't, make a temporary funnel out of cardboard.
Chip the form off the cement 48 to 72 hours later. The type of form dictates how to remove it. A plaster form can be removed with a chisel, a wood form with a pry bar.
Level out a large area of fine-grain sand. It doesn't have to be perfectly level -- just enough to have a flat work surface.
Wet the sand down to the same consistency you use to build a sand castle. The sand should be easily molded and hold its form, and not be runny.
Dig out the design you want to cast with cement, or press an object you want to replicate into the sand, to create a negative image. This process is the same as for casting footprints.
Mix the cement in a 5-gallon bucket.
Pour the cement into the form you created in the sand. If you are working outdoors and there is a chance of rain within 48 hours, cover the entire sculpture with plastic wrap or a disposable tarp.
Remove the sculpture from the sand and wash off any residual sand. If you used a covering, peel it away from the sculpture before you lift it out of the form.
Paint or seal the cement immediately.