This mod and any involving coil taps require your humbuckers to be 4-conductors. That is, they must have four separate wires, one pair for each coil (plus a shield wire, commonly).
As drawn below, this schematic describes a guitar with a single master volume and master tone. This is typical of Ibanez and Steinberger guitars in particular, although it is routinely applied to other makes and models. However, this mod can be applied as well to guitars with only two humbuckers (i.e., without a middle single-coil). See bottom of the page.
Different pickups will have different color codes for the wires. Consult the manufacturer's website for which wires feed from which coils.
In this diagram, the red wires from each pickup represent the "hot" feed. The blue wires from each go to ground. The purple and green wires on the humbucker are the pair that are usually wired together (in series) in the standard humbucking configuration.
In this diagram, the gray wires go to ground. Other colors were chosen arbitrarily. Wherever you see a ground symbol next to a potentiometer, the wire should be grounded to the metal on the back of the pot. After that, all of the grounds should all go to one point to avoid ground loops.
For the sake of simplicity, the string ground is not pictured in this diagram (but you absolutely need to connect it!). See the "All About Grounding" page to learn more.
Note: The image above is larger than displayed. If you wish to print it, save it and resize as desired to take advantage of the actual resolution.
The combinations you end up with are:1) Neck
2) Neck (tapped) + Middle
4) Bridge (tapped) + Middle
There are a couple combinations we could try that drop a coil here or there. Specificially:
Only one humbucker. Your standard '80s "super Strat" only had a humbucker in the bridge position, so you can always employ this mod for simply tapping the bridge coil. Just ignore the purple wire above since there's nothing to tap in that position.
Two humbuckers, no middle. Since many guitars have two humbuckers and lack a middle pickup (e.g., Les Pauls, etc.), just remove that position from the mix. The result will be:1) NeckThe middle position (i.e., #3) will be dead. This has a number of musical and other practical applications. See more about it at the link.
2) Neck (tapped)
3) Silent switch!
4) Bridge (tapped)
And so on... As long as you're under the hood, so to speak, I would suggest a phase switch be added to the middle pickup (if you (still?) have one) via a push-pull to grant still more versatility, but I leave that up to you.