Select a Zener diode with a reverse voltage breakdown specification that is lower than the voltage you are presently using. Use a 5 volt Zener and a 12 Volt power supply for this example.
Subtract the supply voltage from the Zener voltage and then divide the result by the typical Zener diode reverse current to obtain the value for the current limiting resistor. Use a Zener diode with a 5 milliampere (0.005 A) reverse current for this example. Calculate that the resistor value is 1200 Ohms.
Connect the positive terminal of the battery to one end of the 1200 Ohm resistor. Connect the other end of the resistor to the cathode of the Zener. Connect the anode of the Zener to the negative terminal of the battery.
Measure the voltage at the Zener to be approximately the Zener voltage, or 5 volts in this example. Connect one end of a 10000 Ohm resistor to the Zener's cathode and the other end to the Zener's anode. Measure the Zener voltage and write it down. Remove the 10000 Ohm resistor and then replace it with a 2000 Ohm resistor. Measure the Zener voltage and write it down.
Continue placing smaller resistors across the Zener and measuring the Zener voltage. Stop replacing resistors with smaller resistors when the resistor placement results in a voltage reading less than 4.8 volts. Conclude that the Zener diode won't produce a voltage above 4.8 volts for any resistor that has a resistance value lower than the last resistor placed.