Do you have satellite or cable?
I have satellite (Dish Network) but I assume it's the same...normally you just find UFC in the PPV Channels and select it. It will ask you if you want to purchase it, so click on yes. Make sure you do this in advance just in case something goes wrong.
It will usually give you a confirmation on screen.
(or you can just call your cable/satellite provider...but clicking is easier).
You can order PPV right up until it starts