How do you book them?
Can it be done on the disney site or is it best to find a disney travel agent? the high street people dont seem to know much about them
Either book directly or though a Disney specialist TA. Cruising is a specialty in the travel industry, and Disney even more so.
Before you do either, educate yourself about cruising in general and Disney cruises in particular. cruisecritic.com is an excellent site to begin the process. They also have forums for just about every subject relevant to cruising. You can join or just lurk, as you choose! :)