An edger is a small, flat, plate-like tool with one side rounded down. The rounded down edge shapes the top outer corner of a concrete slab when the concrete is still wet.
Handheld edgers are about three inches wide and six inches long with a radius that can be between 1/4 inch and 5/8 inch and a wooden or padded plastic handle.
Walking edgers are larger versions of the same tool but with a long handle, like that of a bull float. The edger that David Odell uses in this video is 12 inches wide with a 1/2-inch radius.
Before breaking out any edger, clean and flatten the concrete slab
Using a hand float, flatten the surface of the slab along the outer foot or so of the concrete. Float the edges with a hand float until the perimeter is flat.
Next, or perhaps first, clean the concrete waste, dirt, and grass from the top of the forms so it will not fall off into the concrete later. Do not allow that waste to be worked into the concrete because it will leave a hole or impression in the slab.
The main rule for using a walking edger: keep it flat
As you slide the edger along the slab, don't focus on the radius edge—or the outer edge, for that matter. Focus on gliding the edger flat on top of the top surface. A clean edge is the byproduct of a flat run; it is not the focus of the process.
You will probably make a line on the outside edge but do not worry about it or focus on it. Concentrate on running the tool flat and straight.
If you focus on the edge, there is a good chance you'll twist the tool, which will cause a rolled edge, lowering it below the adjacent concrete or formwork.
Hand edgers are useful little tools, too
For tighter jobs or small runs, a hand edger is all that's needed, and the process is similar. Again, flatten the edge first, using a hand float or a trowel.
Clean the edge of the existing concrete by flipping the edger backward and running the radius along the side of the existing slab. Now, flip the edger forward and slide it along the fresh concrete to create a clean edge abutting the existing concrete.
After tooling with either the walking edger or the hand edger, IMMEDIATELY clean the line on the wet slab from the outer edge of the tool.
That line is hard to buff out after the concrete has cured.