Plan your sculpture prior to performing any work on the stone. Draw the sculpture onto paper with a pencil, showing the desired shape from as many different angles as possible. Mark the dimensions of the piece on the paper as well to aid in scaling the carving.
Create a concrete form for creating the block of stone you wish to carve. Build a form using plywood for the sides and bottom of the form, with 2-by-4 wood to brace the plywood. Use a measuring tape to mark off the size of the plywood sheets needed, making sure that the dimensions of the cubed form that you're creating are large enough to allow you to carve the size of artwork desired. Connect the form with wood screws at the joints to allow for easy removal of the block.
Mix the Portland cement-based concrete to create your artwork. Choose a concrete mix that matches the appearance of the statue surface you wish to create. Concrete comes with a variety of aggregates, the gravel that helps form the mixture. Choose less aggregate for your concrete mix to create a smoother surface. Combine the concrete with water in a wheelbarrow using a spade until you have a mixture that's smooth in consistency and slightly stiffer than cake batter. Fill the form with the concrete and allow it to dry for 24 hours.
Remove the form from the concrete by unscrewing the wood screws and then prying the plywood away.
Rough out the edges of your artwork with a hammer and heavy chisel. Use the drawing to mark the concrete with chalk as a guide for shaping. Place the chisel onto the concrete and strike the head with the hammer to remove chunks of the material. Use a slow steady pace, taking away the concrete in small sections to avoid ruining the block. Continue to work with the chisel until you have the shape of your artwork revealed from the stone.
Use a point chisel to shape your art with more precision. Continue to remove bits of the concrete, and the many wide lines created from the heavy chisel used to roughen out the artwork. Switch to a toothed chisel to continue to increase the detail work in the stone and to remove the lines created with the point chisel. Switch to the final chisel, the flat chisel. Use this chisel to shape the final size of the artwork and remove the lines left by the toothed chisel.
Use a rasp to smooth any remaining chisel marks and to add fine detail to your piece. Remove the rasp marks with a file to complete the detail work.
Sand the surface of the concrete with an increasingly fine series of sandpaper to smooth the surface. Use wet/dry sandpaper and saturate each piece before sanding. Begin with a coarse 100-grit and paper and work your way up in grits until the surface of the concrete is the desired texture.