Its a very simple job.
Depending on what you currently have will determine what you need to do, I'm guessing you have a single Gamma radio/tape unit?
If you don't have the tools necessary to release the unit currently fitted, go see you nearest friendly dealer (or just buy a set from your local stereo shop, only a few quid).
After removing this, remove the standard fit cage.
Fit your new cage, plug your new loom into the factory fit stuff (it should all be standard ISO connectors, so you can't go wrong) and stuff the head unit in. Hardest problem will be fitting all the wires in!!
Final step is to sit back and enjoy!!
Any probs, drop us a line.