I did this write up a ways back for someone in another thread and it seemed to be helpful so I thought I'd post it here...
Things you'll need:
-"add a fuse"
-2 barrier strips
(from radio shack)
(for your barrier strip, link below)
-Some means of splicing wires together
. I used those things that you can crimp with a pair of needle nose pliers but if you are good at soldering, then go right ahead
-a pair of dikes
-razor blade or knife
-a pair of needle nose pliers
-and last but not least you need some lights
Get your add a circuit Amazon.com: Pico 0956PT 10 Amp Mini Fuse Add-A-Circuit Fuse Holder: Automotive
and plug it in anywhere in your fuse box (Located next to your passengers left foot for you noobs). Depending on how many lights your using I'd go for at least a 10 amp slot.
If you want a switch (I suggest getting a switch) run a wire from the fusebox to one terminal of your switch. Make sure the wire is long enough to reach where you plan on mounting your switch.
Buy two each of these : 8-Position Dual-Row Barrier Strips - RadioShack.com and a jumper 8-Position Jumper - RadioShack.com
And put the jumper down one side of the barrier strip. This will make it so you can have one power source and plug multiple items into it.
Take a wire and run it from the other terminal of your switch to one of the connecters on the duel row barrier strip on the side that you install the jumper.
So at this point you should have the add a fuse
in the fuse box with a fuse plugged in. From there you should have a wire running to a switch
of some sort. From the switch you should have another wire running to your barrier strip
with a jumper
That was your positive side, now onto your negative side:
Find a ground under your dash and attach a wire to it.
At the end of the wire attach your other barrier strip with a jumper.
So now your positive side ends in one of those barrier strips as well as your negative side. So now you can take your light and just plug the positive into one of the open terminals on the positive side and plug the negative into the negative side and you should have power going to your lights.
Ok Heres a couple things to check for if you are having trouble. First of all, most of the extra fuse slots won't provide power if the key is out and a door is open. So close the door and turn your key to the "on" or "acc" and that should give you power to the fuse box. Another thing that happened to me was I had the wires wrong on the switch so I just switched them around and it worked fine. Other than that I'm clueless when it comes to electronics :P