Oil temperature senders usually come in two forms - the sump plug unit that replaces your oil drain plug and the dipstick sender unit. The latter of these is generally acknowledged as being the better one and there is no risk of 'losing' the unit going over speed bumps etc. Installing the sender is fairly straight-forward, only needing two wires, with the gauge being slightly more involved.

Method - Sender
*Before you start working on car electrics always disconnect the battery*
The first thing to do is to to get the depth and oil level marks off the original dipstick. To do this, take the original out and lay them side by side. Adjust the length of the VDO unit by moving the holder part (the bit that clips on to the top of your dipstick hole) until it matches the original. Next using a sharp knife, mark the positions of the full and low oil level marks taking measurements from the original. Be careful to not press too hard - just enough to mark the surface.
Put this in place in the dipstick hole. Next measure & cut a length of wire that will reach from the unit to an earth on either your engine or where the rear lights earth (behind the black sound deadening if you have it) attatching a spade connector to each end. Put the other end on to the earth terminal on the dipstick.

Now you will have to run a wire from the dashboard area where you have decided to put the gauge to the dipstick. The easiest way is to run this under the carpet and through the grommet under the rear seat where the big lead goes through to the starter and then feed it through a gap at the back of the engine bay. At the front push it up past on of the plastic grommets in the doorpost behind the carpet. Once you've sorted out the path of the wire connect one end to the dipstick. Now it's on to the gauge...

Method - Gauge Fixing
First of all you have to decide where to put the gauge. You can attach it to the underside of the dash by either screwing it on or using a velcro pad, put it it the radio hole (although some cutting of the dash will be needed) or in one of the speaker grills either side of the speedo. I chose the speaker panel. If you do this you will need to take the panel off. This is held on by four lugs - one in each corner - which need to be straightened with the pliers to allow the panel to be removed - you'll know what I mean when you see them. Mark out the position of the gauge on the panel, remembering to take the measurement of the body of the gauge and not the lip at the front. Then carefully cut this out with the circular cutting disc - the sort you get from a model shop. Check that the gauge fits in the panel correctly, then put the panel back in the dash (with the gauge already in it).

Wiring up
What the gauge will need to work is a positive and a negative connection, plus the wire from the sender unit and one for the lighting. Looking at the spade connectors. Beside these you should see some symbols which identify which is connected to where. Firstly you need to identify a positive feed from the fuse box. This must be live only when the ignition is on (otherwise your battery will go flat as the gauge will constantly be on. To find the correct wire, either identify from a wiring diagram which is the wiper motor feed or another suitable connection. To test to see if this is only live when the ignition is on first reconnect the battery. Now taking the electrical tester, connect the crocodile clip to an earth and place the other end on the metal of the wire you want to find out about. If it lights up the bulb in the tester, you want a different wire. When you have found a suitable wire, turn on the ignition until the warning lights on the speedo come on and then repeat the test with the tester. What you want is a connection that lights the bulb only when the ignition is on.
Once you have established this, disconnect the battery again and using a 'piggy back' electrical connector (this is a female spade terminal with a male attached to its back), connect it to the positive terminal on the gauge. For the negative, there is an earthing cluster to the right hand (looking from the front of the car) side of the fuse box. To get lighting on the gauge, take a feed from the lighting for the speedo, and last of all connect the sender wire.
What you should have now is a working temperature gauge which will let you know if you are over-heating on a long run or if the temperature increases suddenly.

Generally, normal running temperature is around 100 - 120^oc