when to and how to!

Earth's magnetic field

Earth's magnetic field (also known as the geomagnetic field) is the magnetic field that extends from the Earth's interior to where it meets the solar wind, a stream of charged particles emanating from the Sun. It is approximately the field of a magnetic dipole tilted at an angle of 10 degrees with respect to the rotational axis—as if there were a bar magnet placed at that angle at the center of the Earth. However, unlike the field of a bar magnet, Earth's field changes over time because it is generated by the motion of molten iron alloys in the Earth's outer core.
How to Use a Compass & Map [Compass Navigation Tutorial]

about compasses

There are different types of compasses, the magnetic compass is the most common. There is also the gyro compass, the solid state compass, and the astro compass. But in this case you only need to have a magnetic compass.