Have a read in the tuners section of haw to install analogue and digital tuners at the same time, this is the core of what you are doing. If you want the guide setup for all Foxtel channels you will need to read that post as well. Things have changed a bit in the guide area recently with Free*EPG no longer doing the combined guide for you, but all the info and tools are around. (personally I never added a full guide when using Foxtel).
Basically you will:
1. Connect up your cabling
2. Set the STB to output SVideo signal (make sure you have an output SVideo cable and use the correct SCART port)
3. Take an image of your MCE just in case you break it big time (possible at the first attempt - I know I broke it a few times!)
4. Add the 3rd tuner to your MCE
5. Add the analogue channel (Foxtel)
6. Use happily all the new channels and still have nothing decent to watch
