I've just had to replace a cv boot on my A3. It's a bit of a pain, but can be done in a couple of hours.
I should have taken some pictures, Sorry
But can give a rough guide, it's not that difficult just fiddly and typically the lower the cost of the part the harder it is to replace. Not even sure if its the best method but it worked for me anyway.
CV boots, come in a kit form. Got mine from Eurocarparts, £10 each for the kit, this includes a Boot, clips, bolt and grease. You will need both the inner and outer, if you are replacing the outer boot and only the inner if that one has gone, basically to get the outer of one you need to remove the inner one to get to it.
Also while you're at it, you may as well change the wishbone bushes. THey are very easy to get too once the suspension arm etc is out the way.
Basically you need to remove the drive shaft to get to the boots.
1) If you have a wheel with a centre cap, remove this. using a 30mm socket undo the centre bolt holding the prop shaft, then remove the wheel and jack the car up.
2) Compress the springs( not sue if this is really required but certainly helps and stops the risk of the shocks kicking back at you and undo the top suspension mount.
3) Undo the the steering control rod and ARB
4) Undo the 3 small bolts in a V shape holding the lower wish bone on.
5) Undo the brake caliper, abs sensor and secure up so not to damage the pipe.
6) Remove the whole suspension arm including damper.
7) The other end of the prop shaft is connect to the gear box by 6 Hex screws, Undo these with the appropiate allen key with an extension. You may need to turn the engine over as only some of the bolts are accessible at once.
8) once the 6 bolts are undone you can now remove the prop shaft.
9) At the end which goes into the gearbox there is a metal dome cap, with 3 clips fold over the end of the prop shaft, Prise these open and remove the cap, noting that the dents they are located in.
10) push the prop shaft up to expose the end of the shaft, remove the grease and you will see a C clip on the end. Remove this and gently tep the end of the prop shaft off. It's on a teethed spine and will come off with a gently tap.
11) not you can remove both the CV gaitors and replace using the clips included and the grease supplied.
12) putting back to together is just the opposite, Howver don't worry too much about the domed cap mentioned in stage 9 being as tight as it was when you took it off as it's compressed when you bolt the shaft back up.
If I've missed anything sorry but hopefully it'll get you started.
