The resistance of the grounding electrode is metered, as you can see in the following figure, leading the current in the ground through the electrode EI and measuring the voltage VG located at the second electrode EV.
The electrode EI must be placed very far (distance a in the picture) from the grounding system that you are testing, to avoid interference between the testing current I and the system.
Because of that reason, the distance d between the grounding electrode and the EV electrode must be at least five times the size of the grounding system. After you put the EV electrode in the ground you should always check if that point already show a residual voltage, because in this case the measurement won't be accurate. Look at the following figure for reference. An optimal way to check if the electrode EV is in the right point is to do different measurements changing gradually the position of the electrode until you can see no more variation in the measured voltage.
You can do this for every electrode in the grounding system to see the resistance of the system at different ports, results have to be similar or else the grounding system is not good/under failure. You can see required values of resistance for grounding systems in the standards adopted in your Country or region (e.g. CENELEC, IEC, CEN, ISO).