Make sure the car is warm before you start
Put it into service mode - start the car first, then put in the paperclip (otherwise it is in diagnostic mode).
To locate the CO pot:-
open the glovebox, swing it all the way down by easing the catch past the lip on the dashboard, so it swings right down. If you haven't already emptied the glovebox, it will do it for you at this point :D .
Locate the ECU (metal box, approx 7" x 4" x 2" with a couple of multi-pin plugs in the right-hand side as you view it from the passenger seat/footwell).
The actual CO pot is a blue rectangular thing secured to the ECU frame by one of the Philips screws which holds the frame to the dashboard. It is about 2" x 1" x 1/2" and had a plug on one end which has 2 (Turbo) or 3 (NA) wires going into the loom.
On the CO pot is a small slotted screw which is the adjustment.
There are no 'end stops' on the screw, so it will keep on turning, past where the movement makes any difference to the settings.
There are about 20 full turns of adjustment in the pot, so a small movement may make little difference. (just remember to count how many turns you have made so it can be put back as it was if you need to!)
Personally, I found it easiest to remove the pot from the frame, adjust it, and then re-fix it again, but if you have very dexterous fingers, I suppose it is possible to do it all in situ without moving the ECU.
EDIT ADDED: Turn the screw clockwise to make it richer/raise the CO reading, anti(counter) clockwise to make it leaner/lower the CO reading. (Fredjohn Aug 2016)
I used a Gunsons Gastester digital, which is straightforward to use, but takes about 10 minutes to 'warm up' before you can do any adjusting - just the right amount of time to check/re-set the base timing and engine idle speeds.
Once it has warmed up, and is calibrated, the readings are very accurate. The box says + or - 0.5% CO, but I did mine just before taking it to the MOT garage, and set it to 2%, their (calibrated) machine says it was 2.1%.
Remember that current regulations for the UK state that the CO level must not exceed 3.5% for the SE/NA, and 1200ppm hydrocarbons.