Step 3: 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.