Assuing the fan itself is not working, first find its wires under the centre part of the dash. The fan is on the bulkhead so the wires run forward, quite easy to spot. There's a white plug - separate this - should be two or three thick wires. Blue, and/or red (depending on model), and black. Test for 12V between the blue and black, or red and black, wires when you operate the switch (ignition on).
If there is no voltage, check the fuse marked with a fan symbol (not a fan within a box; that's the radiator fan).
Then remove the switch from the heater controls panel, and test that. Mk2 versions, I think the round knobs pull off. Mk1, use a heat gun or hairdrier to heat the four little triangular knobs, then pull them off. Undo two small Philips screws and tease the panel off.
If you did have voltage at step 1, then remove the heater box from the car. It's not quite as hard as you might think! The dashboard can stay in place - there is a square plug in the dashboard top surface and behind this plug, you find the bolt that holds the heater box in place. There are four 10mm nuts on the bulkhead. The floor console (if fitted) must come off. If you are taking the heater apart - which you are, to change the fan - you must take the controls panel and centre console off first.
Also, you need a large plastic bowl to drain the coolant.
With the heater unit out of the car, you can split the two halves to release the fan. It would probably pay to have a spare complete unit.
-Alex