A military lensatic compass allows you to determine the azimuth of a given direction or to determine that direction in magnetic degrees. You can use it by operating and observing the compass's rotating bezel, following the azimuth by dead reckoning.
Look through the sighting slot to read the degrees
Hold the sighting slot at a 45 degree angle over the floating dial. Turn the compass cover up to be able to look through the sighting slot as well as the compass cover's sighting wire. Look through the magnifying glass that is just below the sighting slot to read the degrees on the floating dial.
Center the sighting wire to read the azimuth
Look through the rear sight to sight on an object. Center the sighting wire on that object. Hold the compass steady and keep it level as you look through the magnifying glass at the dial. Read the azimuth aligned with the black index line to know the magnetic azimuth between you and the object.
Set an azimuth to perform dead reckoning
Rotate the compass until the index line is on the desired azimuth, keeping the compass steady and level the whole time. Rotate the bezel ring to match the short luminous line with the luminous magnetic arrow. If you keep the line and arrow aligned, you will be able to use dead reckoning to move forward in the correct direction.