Tools/Parts List
[spoiler]Soldering Iron/Solder
12v Wire
Hot Glue/Gun
Drill/Drill Bits
Key Switch (Read first paragraph)[/spoiler]

First you need to find or buy a key switch. You can search key switch on Ebay and you will be able to find plenty of them I bought mine off Ebay for about $7. The key switch should have two slots to place wire into some have more than two, but you want one with only two and it should look something like this. [spoiler] [ Register or Signin to view external links. ] [/spoiler]

Now that you have a key switch you need to take apart your Xbox case. (This will void your warranty.)
[url="http://www.youtube.com/watch?v=Uj2A7mGGjuY"]Fat Xbox Tutorial[/url]
[url="http://www.youtube.com/watch?v=HgE06dqy8MM"]Slim Xbox Tutorial[/url]
If you need a more detailed tutorial just search Google/YouTube

Next you need to take the motherboard out of the metal shell.
[url="http://www.youtube.com/watch?v=anNmu6Qk2bE"]Fat Xbox Tutorial[/url]
[url="http://www.youtube.com/watch?v=0hj4sp_jPj8"]Slim Xbox Tutorial[/url]
Again if you need a more detailed tutorial just search Google/YouTube

Now you need to get two small wires to solder onto the bottom of the motherboard under where you plug in the power cable. If you can't find them you can cut open an ethernet cable you don't use and there will be four pairs of wires just take one pair. (I use the solid orange/striped orange pair for my lock.) You will need about a foot of each wire to start just in case you cut through the copper when your stripping off the rubber lining.
The wires will look something like this.
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Now you need to strip about 1/4 of an inch of the rubber lining off the end of the wires where your going to solder onto the motherboard.

Heat up your soldering iron and solder each wire onto a seperate solder point under where you'd plug in the power cable. You might want to add a little hot glue after you soldered the wires on just to make sure they don't slip off.
This is where you would solder on the fat Xbox I don't have a picture of where on a slim, but like I said it's right under where you plug the power cable. Sorry the image is a little blurry. For the slim i used solder points 12 and 9.
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Now however you want bend the wire up to be on the top side of the motherboard. You can do this either through one of the holes on the motherboard or in between the motherboard and the fan.

Now you can put the motherboard back into the metal shell, but make sure to be careful to keep the wires from coming off the solder points.
It should look something like this once you put the shell back on.
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Now you can put everything back into your Xbox like the fan/dvd drive/ect.
Now it should look like this if you have a fat Xbox.
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Now you'll need to drill a hole into the top part of your case wherever you want the lock to be. I had to use a 3/4 inch bit to fit my lock in, but it may be different for your lock. If you can't tell which bit you should use start with one you think is a little smaller than youll need and then gradually go up until you the bottom of the lock goes through the hole but the top is larger than the hole.

Now your key switch probably came with a washer put that over the wire and then pull the wire through the hole you made in the case. (So the wire should be through the case, but the washer should remain under it and around both wires.)
The wires should look something like this, but do not put on the bottom part of the case yet.
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Now cut the two wires so that there is about 2-4 inches coming out of the hole. This way you will be able to take the top part of the case off a few inches to take the wires off the key switch if you need to take your case off in the future. Then strip off about 1/2 an inch of the rubber lining without cutting the copper.
The end of the wires should look something like this.
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Now take one wire and put 3/4 of the copper end through one of the holes on your key switch. Take the remaining copper sticking out and wrap it around the bottom part of the copper so that it won't come out of the hole. Then do the same thing with the other wire in the other hole.
Now the wires and your key switch should look something like this.
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You may want to wrap a little electrical tape on each wire/key switch hole to make sure the copper doesn't touch each other. If you don't and the copper does touch you will get RROD every time you turn your Xbox on no matter which way the key is turned.

Now take the wire and key switch and push them through the hole in your case. Then take that washer and turn it about 3/4 of the way on the key switch.

Now you should take a hot glue gun and put a little bit of glue between the top of the key switch and the top of the hole in your case. You have to do this fast so the glue doesn't harden before you push the key switch back down.

Then clean off any glue that's coming out around the key switch and then finish tightening the washer as best you can with your hands.
Once your done with all this your key switch should look like this.
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Now before you put your case back together plug in your power/av cables and your ring of light. Turn on your console if you don't get RROD then turn it back off. Then put the key in and turn it the other way. Then turn the console back on and you should get RROD. If you don't you need to make sure that the wires are soldered on well on the bottom and the the wires are on the key switch properly.

Once your key switch is working you can put the case back together. Then put your Xbox back where you usually have it plug in the cables and either lock it or leave it unlocked and hide your keys.

Proof:
RROD
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(Note: My Xbox has a blue ring of light so it's blue ring of death not red )

Working
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Disclaimer: I am not responsible if your Xbox breaks as a result of this I did this to my JTag and it works for me.