Step 5: Power
Your power supply can be from pretty much anything. An old PSU from a computer, a leftover hard drive power brick; I prefer small and compact power supplies such as those in cordless phones. Laptop power supplies are an amazing power source, they are super cheap and available, usually can supply 3-5 amps (which means a max of about 230 LEDs), and are regulated [which are normally super expensive] at 12v DC. No matter what the plug on the end is, you can bet there are two wires in there: Positive and Negative.
I obtained three 9 volt power supplies at my local goodwill for $3. eBay also has a plentiful selection of power supplies, though it will take some searching. The supply needs 350mA or more to power 18 LEDs. mA determines the maximum amount of LEDs. 99% of 5mm LEDs use 20mA each, so just multiply the number of LEDs by 0.020A (18 LEDs * 0.020A = 360mA, which is technically overloading it, but it still works).
The speaker wire will carry the electricity from the power supply wires to the LED wires. Decide right now which of your two speaker wires will be your positive, and which will be your negative.
As a general rule, the red, yellow, white, or lined wire is the positive, while the solid (usually black) is negative. After the next step, you will have two LEDs and a resistor soldered together. Just try touching them both to the speaker wire, whichever lights up for you is your correct solution.
This step is optional. It makes it easy to plug and unplug the light bar from the power supply. If you don't want to use Quick Disconnects and want the power supply always attached, just twist the wires of the speaker wire and the power supply together and solder them. Wrap them with electrical tape to keep anything from shorting out.
Quick Disconnects have a wire going into them, and usually you just crush it with pliers (Called crimping) and the wire stays in place. An optional purchase to skip this awl & solder method is a pair of crimping pliers. Regular pliers were incapable of doing the job for me, so I pierced and soldered them into place. Knowing from experience now, I strongly recommend just buying crimping pliers than this next awl + solder method. If you don't feel like spending money, then go for this method, which actually is a stronger connection than crimping alone.
Because its not possible to stick solder into the Quick Disconnect (its covered in plastic) and melt it and the wire together, you should coat the end of your wire with solder. Stick the wire into the quick disconnect. Now hit the Awl on top of the Quick Disconnect, this will pierce the casing and squish the metal and wire together. Follow it up with sticking the tip of your soldering iron into the hole to melt the solder. You should have a very solid connection between the wire and Quick Disconnect. Finally, wrap any exposed areas up with electrical tape.