Wiring LED lights can be a bit tricky because of the accuracy demanded in circuit values and the number of LEDs in a typical lighting circuit. With LEDs, you need accurate values for current, voltage and resistance, or the circuit will not work at all. You also need to make a large number of clean connections using delicate parts (as opposed to incandescent and fluorescent lamps, which require fewer bulbs and use lamps that are easy to connect to the power supply). Fortunately, the calculations are pretty straightforward, and LEDs are tough enough to stand up to soldering with little risk of failure.
Gather the LEDs that you want to wire into your circuit. Most LEDs require little power, so assuming you are using a decent power supply, you should be able to put in as many as you want.
Note the specs of your LEDs. Commercially available LEDs have two values you need to be concerned with: voltage and amperage. For the sake of simplicity, use LEDs that all have the same values.
Decide whether you want to wire your LEDs in series or parallel. In series circuits, the LEDs' voltages are added. So, for example, if you have three 2-volt LEDs, you'll have a total voltage drop of 6 volts. Since you need a power supply that is higher than the total combined voltage of the LEDs, wiring in parallel is almost always a good idea. If you're using only a few LEDs, however, you may want to wire in series for the sake of simplicity.
Go to an online LED circuit calculator and plug in your values to determine what resistor you need. Click on the tab that says "Parallel LEDs" and input the voltage of your power supply, the number of LEDs you're using and the amperage and voltage of each LED. The calculator will tell you what resistor you need and the current rating that resistor requires.
Wire your resistor into the circuit. You can either use a resistor rated for the current of the total circuit and wire it to the positive terminal of your power supply or wire a resistor of the same value, but with a lower current rating, to the cathode (the terminal near the flattened edge) of each LED. For example, if the calculator tells you that your 30 LED circuit needs a 100-ohm resistor rated for 6 watts, you can get one 6-watt 100-ohm resistor, or 30 smaller, common 100-ohm resistors (usually rated for 1/8 watt).
Wire the cathodes of each LED in parallel. Hook up each cathode to the common resistor. If each has its own resistor, wire each resistor to the positive terminal of the power supply.
Wire the anodes (the other terminal of the LEDs) to an SPST switch. Wire the other terminal of the switch to the negative terminal of the power supply. You now have an LED light circuit.
A 12v DC power supply is good for wiring LEDs, but anything that provides 5 volts or more of direct current should work fine for most LEDs. Buy your LEDs in bulk online rather than going to a hobbyist store. The cost of LEDs is often greatly inflated in stores.
LEDs often have a "max" or "surge" amperage or voltage. You don't want to use this as the value for your circuit, since it will decrease the life expectancy of your LEDs. Never work on a circuit while it is plugged in.