PreparationDress appropriately. Wear safety glasses http://debauchedchoirboy.tumblr.com/post/122201848427/ho-s-ng-che-p-gas-w-r-d-w-d-e-t-rs or goggles and gloves to protect eyes and hands from flying debris. Jeans, long sleeved-shirts and boots protect legs and arms from flying rocks and cuts from the nylon string of the weed wacker.
TrimmingMow your lawn before you trim. Begin trimming at the outer edges of your property and work inward. Trim around the house foundation, sheds, trees and other landscaping. Hold your weed whacker at a 30-degree angle as you trim. Cut the weeds or longer grass down to the same height as your mowed lawn.
EdgingEdging adds a clean, manicured look to your lawn. Hold your weed wacker sideways to make straight, precise cuts along walkways, paths and sidewalks. Avoid trimming too low as doing so will leave your yard with a hacked, uneven appearance.
Source:GrassTools: How to Use a Weed Eater
All About Lawns: How Do I Trim