The information below is adapted from the U.S. Army Field Manual FM 3-05.70.
If you have an MK-3 signal mirror, follow the instructions on it’s back. An alternate, easier method of aiming the signal mirror is to catch the reflection on the palm of your hand or in between two fingers held up in a “V” or “peace sign.” Now slowly move your hand so that it is just below your aim point or until the aircraft is between the “V” in your fingers, keeping the glare on your palm. Then move the mirror slowly and rhythmically up and down off your hand and onto the aim point as in the figures below.
Wear the signal mirror on a cord or chain around your neck so that it is ready for immediate use. However, be sure the glass side is against your body to protect the mirrored side
Do not direct the beam in the aircraft’s cockpit for more than a few seconds as it may blind the pilot.
Haze, ground fog, and mirages may make it hard for a pilot to spot signals from a flashing object. So, if possible, get to the highest point in your area when signaling. If you can’t determine the aircraft’s location, flash your signal in the direction of the aircraft noise.
NOTE: Pilots have reported seeing mirror flashes up to 160 kilometers (96 miles) away under ideal conditions.