First one creates a p or n type doped sample of Si. By firing accelerated electrons at it, defects are formed inside the sample. Now EPR is done on this sample.
Photons with a fixed energy are shot on the sample, which is subject to an external magnetic field (put the sample between two magnets). By tuning the magnetic field, two resonance peaks (for s=1/2) are found where the absorbtion is maximal. By analysing these two energies one can deduce the g factor (which is given on y axis of these graphs).
However the g-factor in solids is a tensor, so one must rotate the sample and carry out the experiment again sufficient times to get the whole tensor. Here the graphs show the deduced g factor with respect to the angle by which the sample is rotated.