Summarize:

In most situations, this is the optimal length of wire to use to generate the most cutting power. Crank the starter cord to start a gas-powered weed whacker or simply press the “On” button to start an electric tool.  Note that you may need to prime a gas-powered weed whacker in order to start it. Check your owner’s manual for specific instructions on how to do this with your particular device. Be sure to hold down the “kill switch” throttle in order to start the device. This is both the most comfortable and safest way to hold your weed whacker while you’re using it. Be sure to keep the trigger end at waist level at all times.  Watch out for any strains in your wrists or back while you’re using the weed whacker. If you notice any strain, stop and rest for a few minutes before continuing. If your weed whacker comes with a shoulder strap, put the straps over both arms and clip the weed whacker to the bottom of the strap to take some of the weight off your arms. Don’t try to cut the weeds all the way down to the ground; you’ll just end up slicing dirt and wasting your cord. If the grass needs to be leveled out after you finish trimming, break out a lawn mower to finish the job. Try to keep the weed whacker as level as possible when you lower it to the ground. Otherwise, your trimming job will come out uneven. Move the weed whacker forward in the direction in which the wire spins. For instance, if the wire spins in a clockwise direction, move the weed whacker forward from left to right. Use a steady side-to-side motion to cut with the weed whacker as you slowly walk forward. Gently move the weed whacker from side to side without swinging it. This will not only give you better cutting results, but is also much safer. This is where the force generated by the weed whacker will be the strongest and hence most effective at cutting plants. Don’t try to cut large swaths of weeds with the whole wire; this may cause the engine to overload. This is especially important if you’re cutting tall, thick, or densely clumped together weeds, since these will be very difficult to cut. Turn the tool 90 degrees and hold it very firmly to avoid dropping it. Move the spinning wire along the side of the hard surface to create a “gap” between the surface and the surrounding plants. If this is the first time this particular area has been edged, you may want to slightly cut into the dirt to create a more prominent gap between the surface and the grass.
Pull out 6 inches (15 cm) of wire and start the weed whacker. Hold the weed whacker with 1 hand on the trigger and the other on the handle. Lower the head to about 1 inch (2.5 cm) above the ground. Move the weed whacker side-to-side about 1 foot (0.30 m) at a time. Aim to cut with the tip of the nylon wire. Tilt the weed whacker on its side to edge along a hard surface.