A compass may be any device that indicates the magnetic North Pole. It usually consists of a magnetized needle that points to magnetic north. The needle may be suspended in a liquid that allows the needle to turn freely, but is viscous enough to the damp the needle's motion. Some compasses may rely on electromagnetic induction for the dampening effect. A lensatic compass has a magnifying lens on the rear sight to allow a more accurate reading of the dial. This type of compass is often used to orient a map.
Determine your local declination angle. This is the angle between true north and magnetic north. The declination angle varies by location and is usually provided by a recent map of your area. For example, a declination angle of 5 degrees means that magnetic north is 5 degrees clockwise from true north.
Set your compass needle to point to true north. A lensatic compass typically allows you to rotate the compass needle to true north and lock it so that it always points to true north. If the declination angle is 5 degrees, you would turn the needle 5 degrees counterclockwise and lock it.
Lay the map on a flat surface and completely unfold the lensatic compass. Put the compass on the map so that the long side is parallel with the north/south gridline of the map.
Hold the map and compass so they can move together without slipping. Rotate the map and compass carefully until the needle is pointing at 0 degrees. Your map is now oriented toward true north.
Line up the compass to point to your desired destination and note the compass reading. This indicates the heading you need to maintain to reach your destination.