Buy directly from
www.freesatfromsky.com or one of the participating retailers, I have no exclusive codes at the moment so you will pay £75.
Once purchased, you will receive a pack, which explains the next step. What you do is phone Sky up, quote the reference number in the pack and tell them which 4 entertainment mixes you want (you are not tied to a specific 4). They will then book an installer visit with you (usually within a week) who will bring all the required satellite setup, including the standard Sky digibox, which will be a Thomson or Amstrad. The viewing card will be posted to you from Sky directly and should arrive prior to the installer visit.
You can request the installer to add a quad LNB and additional cable, but he's not contracted to do it so be polite and offer him a tenner, or unlimited tea and biscuits.
If you decide that you do not want the subscription any longer, and just wanted to benefit from the cheap satellite setup, make sure you cancel via phone and email within 3 of the 4 months, as you are on 30 days notice and any later than 3 months is too late to avoid further charges.
If you decide to stick with the subscription (80% do!) then you can of course buy a Sky+ or HD box from Sky, or from an independent retailers and connect up yourself, as hopefully the work will already have been done.