While prepackaged devices exist to regulate low DC fixed voltages, it's possible to build your own from scratch. Zener diodes make good low-voltage, low-current regulators by themselves. In beefier power supplies they act as a voltage reference controlling one or more transistors that can handle more current. To illustrate how a Zener works, you can make a simple regulator using a 12-volt, 5-watt Zener diode that will supply up to 300 milliamps of current.
Note the band on the body of the Zener. This marks the diode's cathode side. Since Zeners regulate by reverse conduction, you wire the cathode to positive power.
Turn the unregulated power supply off. Connect its positive and ground to the breadboard's power buss.
Insert the Zener into the breadboard. Insert the 40-ohm resistor into the breadboard so that it connects to the Zener's cathode. Wire the free (unused) lead of the resistor to positive unregulated power from the breadboard's power buss. Wire the power supply ground to the Zener's anode. Insert two longer jumper wires so the end of one connects to the Zener's anode, and the other wire connects to its cathode. For now, leave the free ends of these wires unconnected.
Set the multimeter to read DC volts. Clip the multimeter's positive (red) lead to the long jumper wire coming from the Zener's cathode, and the multimeter's negative (black) lead to the wire coming from the anode. Turn the power supply on. You should read a steady 12 volts.