Most UI replacement packages are made up of a combination of addons obtained from a site like this one with a pre-set configuration of each addon. Making a full replacement from scratch (unit frames , action bars, raid frames.....) would require a great deal of work.
My advice start by by knowing exactly what piece of the puzzle you need ie. what are you trying to do that you can't do (or can't get done the way you want) using an addon already available (try different addons in the same category or ask here as it's more than likely somone has had your requirement already).
Then... see reply 1
