In this Instructable I'll show you how to make a remote lag switch that's embedded in your controller, allowing you to trigger it without lifting a finger.
The "remote" part of this Instructable stems from the fact that the trigger is away from the actual switch, so that you needn't sit within reach of the ethernet cable it controls.
The "controller-embedded" part uses the battery shell from a wireless Xbox 360 controller, so take note that you'll want to have a Play & Charge kit to use the controller. The rechargable battery in the kit replaces the battery shell that comes with the controllers, so most will have no use for it, and the cable will supply power to the controller whilst using the lag switch.

You can also adapt this Instructable for use without a controller, simply by placing the trigger in another enclosure of some kind. I've thought of placing it in a box by my foot, so I can just "stomp" on it when I want some lag.

Step 1: Parts and Tools

This Instructable is a fairly easy one, requiring only a few common tools and some cheap parts from your local Radio Shack or equivalent.

Tools:
Wire cutters & strippers
Soldering iron
Solder
Screwdriver(s)
Drill
X-acto knife/Stanley knife
Multimeter
Hot-glue gun
Hot Glue

Parts:
Ethernet cable (the length is up to you, I used a short one as the distance between my router and Xbox is tiny.)
Enclosure/project box (again, size is up to you, but smaller is better in this case.)
Heatshrink tubing
Veroboard
Spare Xbox 360 controller battery shell (it won't ever hold batteries again, make sure you aren't using it.)
Generic hook-up wire
Shielded wire (we'll be using the shield as another wire, feel free to substitute a different type of wire.)
Two watch-battery holders (CR-2032, CR-2025 or CR-2016 recommended.)
Two watch-batteries to suit the holders
A normally-open push button (I used a button that had both N.O. and N.C. but it's not necessary.)
A SPDT (single-pole, double-throw) 6 volt relay

All up, this should cost you under $15.

Step 2: Modifying the Controller Battery

Let's start by cutting away some of our battery shell so that we can mount our trigger.

First, place the battery housing in the controller and hold it as you normally would.
With whichever finger feels the most comfortable, touch the battery housing and feel whether or not you'll be able to apply pressure to that spot without much effort. This is where we'll put our switch. Mark this location for drilling later.
I'm right-handed, so mine was about 1/7th the way up the shell, on the right-hand-side of the controller, and it was comfortable to press this area with my middle finger.

Using the mark you made, flip over the housing and cut down to the seam approximately a centimeter above your mark. Then cut along the seem and remove the left-over plastic.
Do not cut as far up as the metal tab.

Now drill a hole the size of your switch where you made the mark, and opposite that drill a hole to fit your remote cable.

Step 3: Add the Trigger

Once the battery compartment is cut and drilled, it's time to add the trigger.

The cable connecting to the lag switch must be cut to a length that allows you to be in whatever location you will be playing and still attached, plus some extra slack. I cut mine to be about three metres long, since I'm not that far from my Xbox.

Once cut, strip the cable and solder it to the button, making sure to have a strong connection. The cable may tug on the switch somewhat, so it's crucial that it stays attached. We'll reinforce it in a moment.

Now thread the cable through the rear of the battery compartment and out of the hole.
Place the button in its hole, and if it came with a washer and nut (mine didn't) screw them on now to hold it in place.

To make sure that the button is not put under excess strain from the cable, apply liberal amounts of hot glue to the rear of the battery compartment, stopping the cable from moving.
I also applied some to the base and neck of the switch, to make sure it was insulated and steady.

Once the glue has cooled, sand off any major protrusions and test-fit it back into the controller. It should fit fine, and the button should not hit the back of controller.

Step 4: Ethernet Cable Preperation

Now we're going to prepare the Ethernet cable, ready to solder in a few steps.

Firstly, take an X-acto knife and gently cut away some of the outer plastic on the cable.
You should see some smaller wires inside, make sure not to cut these!

Keep cutting until you have about 5cm or so of removed plastic.
You should see 8 wires, twisted in pairs of both solid and striped colours (this is what gives ethernet cable it's name: UTP cable, Unshielded Twisted Pair). We only need one wire.

The wire you need will either be the solid Green or the solid Orange wire.
There is no need to guess, as I see most tutorials saying, which is silly.
There are two main types of ethernet CAT-5e cable, and as far as I know they only differ in which colors they use to identify the cables.

In order to identify which cable to cut, you need to take one end of your Ethernet cable and hold it up so that the prong (the plastic tab that holds it in place) is facing away from you. From left-to-right count the copper connections. Connection number 2 (see diagram if you're unsure) will have either a solid Green or a solid Orange cable. This is the one you need to cut.

Cut the Orange/Green cable and strip off a few millimetres from the end. Place it aside for now.

Step 5: The Circuit

Now comes the really fun part: building the circuit!

Take a look at the circuit diagram below first.
You'll quickly notice that it's super-dooper simple.
Since the enclosure you'll be using will most likely differ from mine, your circuit will be sized differently, and as such you may need to refer to the diagram in order to understand how to wire it up.

In order to size our board correctly, either measure or look up the internal dimensions of your enclosure, or just eyeball it like I did. It fit perfectly.

The board can be trimmed in two main ways.
The first is simply cutting the board with a hacksaw or equivalent, which yields decent results.
The second, the fast way, is simply to place it in a vice so that the edge is level with the jaws of the vice, and just pull. Or push. The board will snap cleanly (mostly) along the edge of the vice. Turn it 90 degrees and do it again.
Either method will most likely require you to sand down the edge of the board. I'm not entirely sure, but if I were you I'd refrain from breathing in the particles of PCB.

Once the board has been cut to size, try fitting the components onto the board.
Hopefully they fit. If they don't, you can either use a larger board or wire them up off-board.
Assuming they fit, you can move on to the next step, but don't change their layout.

Step 6: Circuit - Preparation

Whilst your parts are still sitting on the board, flip it over, careful to make sure no parts fall off.
Using a Sharpie or similar, carefully trace around the leads sticking through the board, and label them.

If you're using the type of veroboard that has strips running along it, rather than individual pads, you'll want to draw in "breaks" to stop short circuits now.

I drew lines between connections to visualize how it would be wired up.
This is optional, and it made things more confusing, so you might not want to.

Once you've confirmed that the circuit matches the diagram, you can drill-out the breaks.
Get a large drill bit and sit it gently in one of the holes where you want the break.
Hold the board steady and very slowly begin to drill. You do not want to drill more than is absolutely necessary.
Stop drilling once the copper track is removed from that hole. You must make sure that not copper from that track is left on the hole, otherwise a short circuit may occur.

Once the holes are drilled, go ahead and rub some steel-wool across the copper tracks to help them take the solder. You should still be able to faintly see the marks you made.

Step 7: Circuit - Component Soldering

Alrighty.
Place the components on the board in the same way you laid them out before, and begin to solder each in place, being careful not to bridge tracks with the solder.

Once they're soldered in place, add any connectors you need (the way I placed the components I only needed one, yours might require more).

Step 8: Circuit - Trigger Soldering

Once the components have been soldered to the board, it's time to add the trigger wire and Ethernet cable.

Begin by drilling a hole in the top of the enclosure for the trigger cable to go through.
Next, thread the cable through this hole.
Tie a knot in the cable on the inside of the enclosure so that about four centimetres are left.
Strip the cable.

Depending on the cable you used, you might not have to do this, but for mine I did.
Due to the fact I used the shielding as a wire in my lag-switch, I didn't want all that exposed copper touching anything, so I opted to solder two insulated wires to the original cable.

Then solder these two cables to their appropriate positions on the board, that is to say, one side should go to a battery terminal, and the other should go to the unwired power input on the relay.

Once the trigger is connected, and this is optional, you might want to put the batteries in their holders and press the trigger a few times. You should hear the relay making clicking noises. If you don't, go back and check your solder joints. Better late than never.

Step 9: Circuit - Ethernet Soldering and Assembly

Almost finished now!

Take the Ethernet cable you prepared earlier and thread the Orange/Green wires into their correct holes on the board.
Then solder them in place.

Congratulations, the circuit is finished!
But wait, there's a few things we can do to improve it first.

Take the tabs the enclosure came with (presuming you used a similar enclosure) and cut a small portion out of each. The Ethernet cable should fit snug without being compressed into these holes.

Go ahead and glue the board into the enclosure now, and also add the tabs at either end of the enclosure.
I placed a small dab of hot glue on every wire-to-circuit-board connection to make sure they didn't come loose, or place excess strain on the solder joint.

Step 10: Completion and Finishing Touches

Now that the circuit is complete and the board is glued tight, all that's left is to screw up the enclosure. Not screw it up as in break it, but to literally screw-it-up.

Plug the lag switch in-between your router/gateway/modem and your Xbox/Playstation (but not between your modem and router!), and turn on the console. If all goes well, you'll sign in to XBL/PSN fine, and won't disconnect when you press the trigger briefly.

In order to use your lag-switch, start a game of CoD, Halo, or any other generic multiplayer game of your choice with some willing friends, and ask them to run around without stopping. Try briefly pressing the trigger and see if they stop moving, if they don't, do not fret, in some games the player will appear to continue moving in the direction they were going before the lag kicked in. Next, ask them to watch you, and begin to run around. Then press the lag switch, and you should appear to freeze to them.
Also note, you probably won't be able to hear them whilst you are pressing the lag switch.

The lag switch works better in some games than others.
If you hold the lag switch for too long, you might disconnect, or, if you are the host, other players might time out.

I hope you've enjoyed my first Instructable, and I hope you find it useful!
Try not to anger people with it in multiplayer online, as it really does give you an unfair advantage. I only use it in private matches.

Also, as a final closing comment, you may wish to place a piece of electrical tape over the four copper connectors in the back of the Xbox controller, just to stop hot glue breaking off in there or whatever. They are used to connect to rechargeable battery packs, and you can safely tape over them whilst using the lag switch.