Mark the line of the curb with wood stakes and mason's twine according to the type of curb planned. Follow municipal guidelines for a street or driveway side curb, which generally are 6 inches wide and 6 to 8 inches high. Choose your own width for a path or flowerbed curb; these usually are 4 inches wide and 4 to 6 inches high, thicker and higher if decorative finishing will be used.
Dig a base according to the type of curb you plan to install, using a shovel. Make the depth at least twice the height and width of the planned curb. Allow space on either side for forms to hold the concrete, or prepare the area for a concrete curbing machine, according to its directions. Put down a gravel layer the width of the curb and the depth of the height.
Rent a concrete curb edging machine or build forms to make the curbing. Fill curbing machines with concrete mix according to the instructions; some use a dry mix of cement and sand, and you add water to the machine as the curb is being poured into the prepared space. Machines are expensive to rent and can be very difficult to handle. Building wooden forms --- or renting metal ones --- is an easy and inexpensive alternative.
Build forms with 1- or 2-inch lumber the width of the curb height. Set the boards on edge and secure them with wooden stakes nailed to the outside edge. Use 1-inch boards for a small garden border, 2-inch for street or driveway curbs. Coat the insides of the boards with mineral oil to prevent the concrete from sticking. Put thin spacers of wood, metal or an asphalt material between the boards every 18 inches to hold them apart, and clamp the spacers securely with metal clamps or by nailing 1-inch by 2-inch boards across the tops of the forms.
Mix concrete in a wheelbarrow or mixer and pour it into the forms, using a board to tamp it firmly into the entire space. Let it set until water begins to emerge on the surface, and then remove the forms and smooth the concrete with a mason's trowel. Use a special curb edging tool if desired to put a rounded edge on the top of the curb. Remove the wood or metal spacers, and fill the gaps with an asphalt roofing compound. Let the concrete cure at least one full day, and then fill dirt or other material around it.