Select the first location for a concrete curb and outline the area with a rope. Position the rope at least 10 inches from the garden or walkway -- 4 inches for the form boards and 6 inches for the curb width. If you're adding curbs to both gardens and walkways, consider forming the straight curbs first so you can get comfortable with the process.
Dig a trench between the rope and garden with a spade. It should be about 5 inches deep and tamped at the bottom to stabilize the soil beneath.
Lay 3 inches of 3/4-inch aggregate in the trench and tamp it to create a strong base for the curb and improve the site's drainage.
Arrange two-by-fours against the inside walls of the trench, stacked in two layers or to the desired height of your curbs. Nail the boards together with a hammer, and brace the forms around the outside of the trench with wood stakes. The wood boards will form the height and shape of your curb.
Follow the instructions on the concrete mix and stir it in a wheelbarrow. Add color pigment if you want to give the curbs a hue.
Add the concrete into the trench until the forms are filled. Use a float to smooth and level the top. Score lines across the curb every 3 to 4 feet with a trowel to prevent cracking.
Wait one week for the concrete curbs to set, and then remove the form boards. Backfill gaps created by the forms with pea gravel or soil.