You can edge your lawn any time it begins to look untidy, but the best time to edge is whenever you mow your lawn. This is also the most convenient time, since you already have your lawn tools out and are ready to work. The work will be easier, and cleaning up will take less time if you wait to edge until two or three days after the last rainfall.
It's your choice whether to edge before or after you mow. However, edging before mowing allows the mower to clean up loose pieces of dirt and grass that can get into your lawn during edging. Line up your edger between the border of your grass and the sidewalk. Then cut along this border the length of the sidewalk. Once you've finished mowing and edging, go back and sweep up any dirt and grass clippings from the sidewalk.
Sidewalk or lawn edgers vary greatly. One type consists of a pair of grass clippers mounted on a pole so you don't have to reach down to cut. Another type has a metal half moon at the end of a handle. You push the metal into the border between your grass and sidewalk to cut the grass back. A third type of manual edger has a cutting wheel that you roll along to cut the grass; this model works faster than the previous two. If you have a very large lawn, consider a gas-powered edger.
Before using your sidewalk edger, locate any shallow electrical lines or sprinkler system components along the area of the lawn where you'll be cutting. Cutting into electrical lines can cause an electrical shock or electrocution. If you have trees along the path you'll be edging, work carefully around them so you don't damage the bark. Nicks in trees can invite disease and insects into the tree. Always wear safety goggles when using a power lawn edger since it can kick up rocks and dirt.