A CPL works by simply rotating the CPL until you see it affect the scene you are photographing.
All CPLs have their maximum effect perpendicular to the light source. If you point your index finger toward the light source and extend your thumb perpendicularly, the maximum polarisation occurs in the plane as you rotate your hand around.
Forest scenes are where CPLs truly sing. In this setting, the light source is usually from directly overhead, hence maximal polarisation is in the horizontal plane around you.
When using a CPL in any setting, give yourself some time to examine its effects at various angles to ensure you have rotated the CPL to achieve its maximal effect.
When changing the orientation of your shot from landscape to portrait, don’t forget to rotate the CPL as well.
Avoid using a CPL when shooting with ultra-wide angled lenses into plain blue skies perpendicular to the light. This will result in a dark band in the sky which is difficult to correct in post-processing.