Connect the anode of one diode in the diode array to the positive terminal of a power supply. Connect the diode's cathode to the positive lead of the ammeter.
Connect the negative lead of the ampere to the right lead of a 1,000 ohm resistor. Connect the left lead of the resistor to the negative terminal of the power supply. Turn on the power supply and set the power supply's voltage level to zero volts.
Place the voltmeter's positive lead on the diode's anode and the voltmeter's negative lead on the diode's cathode. Check that the voltmeter reads zero volts. Check that the ammeter reading is also zero. Conclude that the diode is off since there is no voltage drop across it and no current that flows through it.
Increase the power supply's voltage level to 1 volt. Read the voltmeter display. Verify that the voltmeter reads between 0.6 and 0.8 volts. Read the current on the ammeter and note that it is in the miliampere range. Note on a piece of paper that the diode is on, since it has a voltage drop near 0.7 volts and current flows through the diodes.
Subtract the power supply voltage, 1 volt, by the voltage drop across the diode, about 0.7 volts, to obtain about 0.3 volts.
Divide 0.3 volts by 1,000 ohms, the value of the resistor, to obtain 0.0003 amperes, or 0.3 milliamps. Write down that the diode on current is 0.3 milliamps. Check that this current is close to what current the ammeter displays.
Turn off the power supply and connect another diode's anode to the power supply's positive lead. Next connect the left lead of another 1,000 ohm resistor to this diode's cathode. Connect the resistor's right lead to the power supply's negative lead.
Turn on the power supply again. Raise the power supply's voltage between 1 and 2 volts and measure that the voltage drop across each diode is in the 0.7 volt range. Measure the current through the resistors. The diodes are on, since there is a 0.7 volt drop across the diode and current flows through the resistors.
Lower the power supply voltage to below 0.3 volts and read the current through the resistors. When you lower the voltage below 0.3 volt you have switched both diodes on, since there is no current flow through the resistors.