For overgrown edges where the soil and roots of the grass have started to grow over your lawn's border, grab a half-moon edger – this'll allow you to get a good crisp edge by removing the excess overgrowth. Use the weight of your body and push down onto the edge, following the line of the plank of wood or string. Cut through the fray bits and soil on the end.
Once you have completed cutting through the fray bits and soil, remove the turf using a shovel and take away your guide (plank of wood or string) to see the firm straight edge you've created.
Before you use the whipper snipper, you'll need to put on your safety glasses and ear protection. It's also a good idea to wear long trousers and closed-toe shoes.
Run the whipper snipper against the edge of your lawn in order to create a precise straight line. When using, know which way the breeze is going so the grass, rock, pebbles or any other debris is flicking away from you.