I snipped this off the web. It's a reasonably good step-by-step description.
It does require practice, but is really useful when the synchros start
wearing. Even with a recently refreshed transmission, I still find myself
double-clutching on the track.
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To double clutch when going from a lower ratio gear to a higher ratio gear:
 a.. Press down on the clutch pedal while taking the other foot off the
accelerator pedal.
 b.. move the gear box into neutral
 c.. release the clutch pedal
 d.. press the clutch pedal down a second time.
 e.. move the lever into the next highest gear and release the clutch
pedal.
 f.. resume pressure on the accelerator pedal.
To double clutch when going from a higher ratio gear to a lower ratio gear:

 a.. Press down on the clutch pedal while taking the other foot off the
accelerator pedal.
 b.. move the gear box into neutral
 c.. release the clutch pedal
 d.. rev the engine briefly
 e.. press the clutch pedal down a second time.
 f.. move the lever into the next lower gear and release the clutch pedal.
 g.. resume pressure on the accelerator pedal.
The object of double clutching is to match the speeds of the various shafts,
gears and splines before engaging the next ratio.

On upshifts, disengaging the clutch while the transmission is in neutral
slows the input components to match the speed of output components.

On downshifts the engine is revved to speed up the input components to match
the speed of the output components.

Experience teaches how high to rev the engine when performing down shifts
into non-syncro gears.

The synchromesh mechanism achieves this speed matching by "dragging" the
components to a matching speed with the synchronizer rings.

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HTH -

Mark Hald
'77 Carrera 3.0
http://members.rennlist.com/hald

This is what I understand it to be:

1) Push clutch pedal in
2) Shift into neutral
3) Let clutch out
4) Rev engine to match RPMS of next gear you want
5) Clutch in
6) Shift into gear
7) Clutch out

-Mike
'86 911

Double Clutching is a technique used by truck drivers and on very early
autos.

Basically, try to think when you have a stick, and you are driving along at
a certain speed in a certain gear. Look at the tachometer. In that gear, at
that speed the engine is turning a certain number of RPMs.

OK?

Now lets say I am cruising along at 50 mph in 4th gear, engine is turning,
1800 RPM.

I am approaching a winding steep downhill, I want to, use a lower gear to
slow the car, lets say second, because my brakes are bad.

Now, experience tells me in this vehicle, in second gear at 50, the engine
will be turning about 4500 RPM. A lot, but better then a wreck.

So, I also know my synchronizers on the transmissions are shot so what do I
do, won't let me just put clutch in and shift down.

Ok here we go to your question, now that you're prepped.

1. Depress Clutch, shift to neutral,

2. Release Clutch, in neutral Rev Engine to 4500 RPM

3. Depress clutch again, shift to second and it slips right in

4. Release clutch and survive.

The reason double clutching made a difference is instead of the synchros in
the transmission having to get gears up to matching speed so you can shift
into the lower gear, the revving the engine in neutral allows the engaged
input shaft get the gears to the speed they will be going to be spinning at
when you shift.

Works both ways up and down, just remember the idea is, when clutch is out
and you are in neutral, (2.) you are matching engine RPM to the RPM the
engine will be at when you change to next gear, which then allows
transmission components to be "adjusted" to the revs they will be turning.