Assume you have a sample of a solid. First you apply a magnetic field in a certain direction wrt to the sample (eg. along the 100 direction). You irradiate the sample with microwaves and find the magnetic field strength for which you have an EPR resonance (absorption).

From this initial spectrum, you can calculate the principal components of the g-tensor. Then you rotate the sample OR the magnetic field and do the experiment again. Now you get a new spectrum from which you can calculate another g-tensor

Doing this many times will give you the g-tensor (principal components) as a function of the orientation of the applied field