The triplet loupe is hexagonal in shape and is covered by two metal plates. The magnifying portion of the loupe will swivel outward. Place your index finger through the opening, curling it around the free crossbar. Place your thumb on the metal plate, bending it so that the knuckle is raised. Bring the loupe up toward the eye that is on the same side as your hand. Rest the thumb knuckle on your cheekbone with the lens of the loupe in front of your eye, keeping your hand steady and fixed in this position. With the opposite hand, bring the item you are holding towards the loupe until it is in focus. This will be just under an inch away from the lens. Remember:  don’t move the hand with the lens; keep it fixed and stable. Happy hunting!

Kassoy is a good resource for loupes.