Dig a 6-inch-deep trench with a shovel, using a laid out garden hose as a guide. Make the trench's outside edge flat and angle the inside edge towards the garden to give the trench a V-shape. Cut through any roots you come across with hand pruners.
Loosen the garden bed's top 6 inches with a rototiller. Unwind the plastic garden edging and lay it along the garden bed's perimeter.
Insert the plastic garden edging's bottom edge into the trench. The curved "V" at the top points toward the garden. Work down the edging's length until it is loosely inserted in the trench.
Cut off any extra edging with a utility knife. Connect two pieces together using the connectors supplied in a stake kit. Slide the connector halfway onto the first piece of edging. Then, slide the next piece onto the connector so the two pieces meet at the connector's center.
Push the plastic edging firmly into the ground with your hands. Keep the edging's front face tight against the trench's straight outer side. Continue pushing until the loop on the edging's top edge is halfway below ground level. This prevents the edging from interfering with the lawn mower.
Drive stakes through the plastic garden edging to hold it in place. Use four stakes for every 20 feet of edging. Keep the stakes at least 3 feet from either end. Hold the stakes at a 20-degree angle with the pointed edge on the edging. Some edging brands have hooks for the stakes to drive through. Use a hammer to drive stakes into the ground.
Pull soil towards the plastic garden edging to help hold it in place. Push the edging out around outer corners and use your feet to pack the soil behind it.