In the experimental set-up there is a generator of microwaves and these waves (with a particular frequency) are sent onto a sample, between two electromagnets that generate a magnetic field. It is then possible to search the magnetic field value that gives transitions at that particular frequency. This leads to absorption peaks in the spectrum.
The next step is to determine the local g-factor from these absorption peaks for a particular orientation of the applied field. Changing the orientation of the applied field and determining the g-factor each time (i.e. the principal components of the g-tensor as a function of the applied field) then leads to the showed pictures.