As the old guide is now missing the pictures, I thought I would do one as mine had gone.
My ABS light had come on intermittently, but when off, occasionally as I was slowing it would feel like my pads were completely shot and was grinding metal on metal in a juddery fashion. This affected the braking efficiency quite alot and so needed to be done asap! It only did this when I started off with the light off. When it was on after initially starting the car everything was fine, and once it came on after the hairy moment, braking was fine, although the ABS system was overiden and would obviously lock up under heavy braking.
Its actually a very straightforward procedure to change one, but some may be a bit cautious with regard to braking systems which is understandable. The price of a new sensor is about £15 incl VAT from GSF, but may vary dependant on where you get it. Obviously this is an MOT failure too, so for such a small price, its silly not to replace it. Anyway, enough waffle, this is what I did..................
1) This will probably be your first indication your ABS is not working, and 9 times out of 10, its the Sensor as they are a very common failure
2) First thing to do, is loosen your wheel bolts, and jack up your car
3) Remove wheel
4) Turn the hub so you have good sight of the sensor, and good access, 
5) Remove the sensor plug, and stow it out of the way, then measure the resistance of the sensor. If you have a code reader, then as I did, you will already know which sensor is at fault. If not, using your multimeter, check the resistance is between 1 and 1.3 K Ohms. If its sky High, (as mine was) very low, or wavering from one extreme to the other, then thats the faulty one. If all is ok, move on to the next wheel. The rear sensors are a little different, and a little bit more tricky. They come with a cable hard wired, and the connector comes through a rubber grommet and in under the rear bench I believe where it picks up the connector. They are also slightly more difficult to work on due to space restrictions in the rear arch.
This is what it should be
6) Some have said that they have had problems removing the securing bolt, what I did was to spray a load of WD40 on it, and take a small wire brush to clean it up. This worked a treat, but to be sure, take a Hammer, and a 5mm Allen Key, stick it in and give it a few bangs to ensure there is decent contact and it wont round the bolt off. Once the bolt is out, the only real way to remove the sensor as I promise it will be stuck fast, is to smash it to bits! This is the remains and new vs old!!
7) Once its all out, you will need to rub the space down as it will be corroded and pitted like below. I rubbed mine down with 400 wet and dry, and shaved a little off the sensor itself with a stanley blade too. Whatever works for you though if you have a better idea.
Once the sensor is in place, stick some copper grease on the bolt. (This is optional, but like my old man used to say, If your gonna do a job, do it properly!)
9) Put the bolt back in, and connect the sensor back up
10) Then stick the wheel back on, Lower the car, put your tools away, and go for a spin!!!
The lamp should go out once roughly 15mph is reached. Once your done, give yourself a pat on the back for a job well done!! 
