Gravel is rather like young children -- good to look at but difficult to keep in one place. Available in many colors and sizes, this decorative and low-maintenance hardscaping material makes an effective mulch and is also suitable for high-traffic and play areas. Left to its own devices, though, gravel will soon migrate into areas where it isn't welcome, such as the lawn, where it can damage your lawnmower. Suitable edging materials include timber, metal, bricks and cobblestones. The one you choose should fit in with your overall garden and house style. Bricks and cobblestones are easiest to lay on curved edges.
Rake gravel away from the edging line with a garden rake, to give you a clear area to work.
Dig a shallow trench along the gravel edge, just deep enough so the edging will stand at least 1 inch higher than the gravel. For example, if the gravel is 2 inches deep, and the edging material measures 5 inches high, dig a trench 2 inches deep.
Peg a line into the ground to serve as a height guideline. Check that it's level throughout with a line level.
Set the edging into the trench, and firm it in place with a rubber mallet, using the line as your guide to final height. Add or remove soil at the bottom of the trench as necessary.
Check that the edging is level before removing the line and pegs. Rake the gravel back in place and smooth it into brick or cobblestone edging to help lock everything in place.