A double pole single throw (DPST) switch controls the connections to two wires at once, where each wire only has one possible connection. In other words, it's like two simple switches controlled by a single actuator. The DPST switch often appears in circuit breakers, where it is used for 240-volt circuits, with each pole carrying 120 volts separately. It can also be used at home to completely disconnect a circuit from its electrical power source by controlling the hot and neutral wires at once, eliminating all possible paths between the disconnected area and the power supply. DPST switches are also sometimes used to control parallel-wired lights or machines simultaneously.
Ascertain that you actually need to use a DPST switch. They are not very common when compared with other switches. If you aren't sure, consult with an expert or a licensed electrician.
Make sure your DPST switch is actually a DPST switch. The letters "DPST" should be stamped somewhere on the switch. Some people confuse a DPST switch with the more common single pole double throw (SPDT) switch, which is often used in three-way switches.
Wire your circuit to the point where you are ready to install the switch. Always use the correct gauge wire. For typical residential wiring jobs involving a standard 20-amp circuit, 12-gauge wire is usually appropriate.
Connect the switch as you would with a simple switch, handling each pole separately. Cap wires as needed. Make sure that there are no cross-connections between the two poles. This kind of mistake is easy to make and potentially very dangerous, depending on the application. Your time will be well-spent double-checking your work.
When securing wires to terminal screws, make sure you bend your wire in the same direction as the screw will turn to tighten (which is almost always clockwise). This will tighten your electrical connection as you tighten the screw.
When working on electrical circuitry, always turn off the power to that circuit prior to any work. Residential electrical shocks can be deadly.