black hardware

Yes, looks much better with black hardware.  I first tried hex-head (Allen) screws: they looked OK but protruded a little too high even with the countersinking.  I finally arrived at #6-32 black-anodized Philips screws, as you can see.  Small detail, but makes it look and feel better, so it must be thus.

construction details

If you choose to place the switches as I did, i.e., poking through the same slot that the 5-way switch used, then it's a little bit tricky to drill the holes through the pickguard.  I used the two screw holes for the 5-way switch, to temporarily screw a block of wood to the back of the pickguard, so that I could drill through.  I used the narrow blade of a pair of scissors to drill starter holes, deep enough that the diameter of the hole notched the plastic on both sides of the slot.  This provided enough of a starter to keep my handheld cordless drill centered, to then drill 1/4" holes for the switch necks.  It was necessary to run the drill at a very slow speed, it has a great tendency to wander in this situation.  (I probably would have done better to take the pickguard to my workshop, where I could use the drill press to do this, but it was entirely viable to do it with a handheld drill.)

The switches don't look entirely bad just poking through the slot, but I wanted it to look a bit more professional, so I fabricated a brass switch-plate which goes on top of the slot and covers it up, with holes that the switches poke through.  I used a bar of 1/8" thick, 1/2" wide brass, available at places like Lowe's, Home Depot, etc..  I marked the center points for the five holes (three 1/4" holes for the switches, and two 3/16" holes for the mounting screws, the latter with countersinking to 1/4" diameter for the screw heads), and drilled them on the drill press.  Then I used a handheld electric jigsaw to saw the bar to length (while it was screwed to a wooden block which I also cut into).  I used a coarse sanding wheel, #60 grit, made for a machine but held in my hand, to round the corners and shape the edges on the piece.  Then I used #180 sandpaper to rough up and freshen the surface all over the brass, to remove oxidation and leave a good surface for painting.

I fashioned another wooden block with nails to hold the piece in position for painting, i.e., raised up in the air, not lying flat on a surface.  I painted the piece black with acrylic nail polish, several coats with 12-24 hours drying time in between; then I sealed the finish with spray-on matte clearcoat, again, a liberal number of very light coats, with lots of drying time in between.

The switch plate is attached with #4-40 screws.  I had hoped to find nuts to fit the existing screws that attached the 5-way switch, but these are some strange thread, not 40, not 32, maybe metric or something?  So these get saved with the 5-way switch (probably better that way); and I use the generic #4-40s.  They are inferior in my view, just because they have a slightly bluer silverish colour; the original screws have a slight yellow cast, which matches all the other silver (chrome) hardware on the guitar.  The #4-40s look a bit cheap and "Home Depot" in comparison.  Hopefully, having them partially countersunk in the black switch plate will reduce the unpleasantness of their appearance, even as it brings attention to all the shiny metal parts through contrast.

But I wouldn't put it past myself to replace these screws with black-anodized #4-40 hex-head screws.  Because, yep, I keep noticing them, and yep, they keep bothering me!

In the photo above, you can see that the switches are in positions 0-1-1.  This is "Strat" tone #2, i.e., treble and middle pickups: one of my favourite settings for single-coil guitars, for many types of clean or near-clean playing.  This particular setting is, of course, available without doing all this extra work.  But once you start sampling the six out-of-phase tones, I think you'll agree with me that no Strat should be without this extended capability.

the circuit diagram, and wiring suggestions

Here's the switch position diagram again, with the circuit diagram added at the bottom:

This is viewed from the back, of course.

The first step is to slacken the strings on the guitar so that you can remove the pickguard.  Slacken them enough that you can raise the whole bunch of strings about 2 inches over the pickups.  If you loosen the strings but keep them still wrapped around the tuning pegs, you can probably re-use the same set of strings.  After the screws are removed, with the strings slack, you can carefully maneuver the pickguard out from under the strings, to the side.

I'd suggest taking one or more photos of the wiring on the back of the pickguard, before you start cutting and drilling.  (Take some when you're done, as well, before you close it back up, so that if something doesn't work, you can get a clue as to maybe why not, and/or get some advice by sharing the photo, before you have to take it all apart again.)

The pickguard will remain tethered to the guitar by two wires: the output coax to the jack, and a ground wire which presumably attaches to the bridge underneath, somehow.  Be careful not to overstress these wires.  You can probably do everything with these wires attached; however if you're not afraid to re-solder things, you can cut these wires to make your life easier.  Cut them right next to the attachment points on the pickguard end, so you preserve maximum length to reattach.

Clip off the wires attached to the 5-way switch, and remove it.  Take the time to prepare the new ends of the wires you have clipped, by stripping insulation to the right length, twisting the strands together, and then tinning each conductor with solder so that when the time comes, it easily and almost instantly connects to the chosen, also-pre-tinned, lug on the switch, with just a touch of heat from the iron.

The three switches can be wired up as a unit separately, before mounting in the pickguard.  If you bend the wires carefully to avoid shorting, you can wire this with bare copper wire, such as #22 solid.  Then once the assembly is mounted, you can solder on the (pre-prepared) coax lines from the three pickups, and the black and red unshielded output wires.

13 is better than 5

This is a modification to increase the number of tonal possibilities in a 3-pickup guitar, beyond what is offered by the standard 5-position "Strat" pickup switch.

I replace the 5-position switch with three 3-position toggle switches, one per pickup.  Each switch has a center-off position.  The other two positions turn on the associated pickup.  The downward position is in-phase, the upward position is out-of-phase.  Thus, every possible combination of pickups and phase is available.

In fact, a number of the combinations are redundant: i.e., they sound exactly the same as another combination.  There are 27 possible combinations, but only 13 useful and distinct tones (plus all-off).

Demonstration of the 13 tones:
https://soundcloud.com/benjamin-ketcham/3-switch-strat-tones

This is effectively the same wiring as Brian May uses on his famous Red Special guitar (except that he uses six 2-position slide switches; I prefer it as the three 3-position toggles).

I'm doing this modification to my new little Squier Mini-Strat.  I originally considered implementing a smaller number of additional tones, not every possibility, through adding some pull-switches to the volume and tone knobs.  Less versatile, but can be done "reversibly", without permanent damage to the guitar.  My method here requires drilling new holes in the pickguard.  I arranged to do this in a "minimal-impact" way, in that the new holes follow the slot of the old 5-way switch.  The toggle switches protrude through these holes and then mount to a brass switch-plate which I am fabricating, and which attaches to the pickguard using the old screw holes for the 5-way switch.  I will save the old switch, and thus one could restore the old functionality, albeit with a somewhat jagged-looking slot for the switch handle; one  would probably want to replace the pickguard.  Good thing this guitar's not vintage!  But I'll never change it back anyway, this wiring is the way to go.

So probably of interest to others, is both the wiring to accomplish this, and how I physically do it so that it looks reasonable.  Do I need to mention this?  Don't undertake a project like this unless you are quite confident in your competence, both to open up your guitar and drill holes into it, and also to make really good solder joints.

The switches are DPDT center-off mini panel-mount toggle switches, which I got from digi-key (.com).  This type of switch is available with several kinds of toggle: rounded, flat, metal, plastic, different lengths, etc.: be sure to look at the alternatives and choose something that will look and feel nice for your guitar.  These little items are pricey, but don't try to cheap-out too much on this.

Here are the 13 "useful" tonal combinations.  Note that I call the pickups "bass", "middle", and "treble".  Many others call them "neck", "middle", and "bridge".  I organize the combinations as shown, mentally, into four different groups:

The dashed line separates the combinations into two major categories, which I call additive and subtractive.  The subtractive sounds have one pickup out-of-phase with the rest; the additive sounds have everything in-phase.

The additive sounds consist of the five standard "Strat" sounds (tones #1 - #5), plus the two combinations you can't get with the 5-way switch alone: treble plus bass (tone #6), and all three (tone #7).  Some of the newer Fender guitars have a pushbutton in the volume knob, which they call the "S1 switch", which allows you to get tones #6 and #7; hence my name for these tones.

The subtractive sounds involve either two or three pickups (flipping the phase on a single pickup makes no audible difference); there are three possible variations for each, so a total of six subtractive settings (tones #8 - #13).

My Squier only has two knobs, but most Strats have three.  I should note that, with three knobs, it is possible to get all these tones without drilling the pickguard, by putting a pull-switch into every knob position.  The logical way would be to put an "S1" type switch with the volume knob (if not already so equipped): this switch turns on the bass pickup, independent of the 5-way switch, thus enabling tones #6 and #7.  For the out-of-phase tones, it's only necessary to be able to flip phase on two of the pickups, not all three, so a phase switch in each of the two tone knobs would do the trick.  As mentioned, I don't have the three knobs needed to do it this way, and I don't mind making permanent mods to my guitars.  Plus, I find the three-switch wiring presented here, to be more intuitive than figuring out how to get the same combinations from a 5-way switch supplemented with pull-knobs.  But if I had a nice vintage Strat, I'd probably think twice.

I thought the all-off setting, #0, might have more hum problems than muting the guitar with the volume knob.  However, at least on this Squier with the amp I'm using, there seems to be a low-enough impedance from the volume pot, that there's no hum in position #0, so maybe my warning about this is needless.