Les Paul and Les Paul-style guitars utilize a three-way pickup switching circuit which allows for selecting between the neck and bridge pickups, or for using both combined. In standard wiring, the "down" position on the toggle switch engages the bridge pickup, the "up" position engages the neck pickup, and the "middle" position uses both. Whether you are building a project guitar or need to replace a damaged toggle in an already wired guitar, the procedure is the same and is actually fairly easy provided the rest of the wiring in the guitar is correct.
Inspect the three-way toggle switch to identify its terminals. There will be one side with four terminals, two for receiving input and two for sending the output to the output jack. The opposite side of the switch will have a single terminal for grounding.
Bend the two middle terminals on the side with four terminals together until they touch. Solder these two terminals together to create a single output terminal.
Mount the switch in the cavity just above the neck of the guitar and secure it in place with its nut.
Route the output wire from the volume control for the Les Paul's bridge pickup to the terminal corresponding to the "down" position on the switch and solder it in place. The terminal used will be opposite the switch position, so the "down" position will be matched to the upper terminal.
Repeat step 4, this time soldering the output from the middle pickup's volume control to the terminal corresponding to the "up" position on the switch.
Solder an output wire from the combined middle terminal on the toggle switch and run the wire through the guitar to the output jack, soldering it to the terminal on the jack.
Solder a wire to the grounding terminal, running it through the body of the guitar to the wiring cavity. Solder its free end to a volume pot chassis.
Test the guitar and switch for proper function by playing it through an amplifier. Hum or buzz may indicate poor or loose ground connections, and scratching when the switch is toggled may mean a loose input or output wire. If these problems are experienced, double-check all wiring and make sure connections are secure.
Special multi-core shielded wiring can be obtained for guitar wiring, allowing for keeping all wire leads in a neat bundle. However, if you do not have access to this type of wire, it is a good idea to group wires together with electrical tape to keep the electronics organized and for easy troubleshooting.
Always solder in a well-ventilated area to avoid the inhalation of toxic fumes.