Most writings on Elemental Dignities do not exactly follow Mathers' instructions. For instance, they often stress 'the missing suit' in triads, which Mathers never did. You can read his original essay in various editions of Regardie's _Complete Golden Dawn_ or in Robert Wang's _Introduction to the Golden Dawn Tarot_.
The best way to understand how it works is to actually lay out the cards from Mathers' examples and follow his analysis closely. I've summarized what he did in my book, _21 Ways to Read a Tarot Card_. Paul Hughes-Barlow has an excellent tutorial on his website, although he doesn't always follow Mathers (and doesn't warn you when he has deviated). Liz Hazel's book is also excellent, with the same caveat.
BTW, you certainly don't have to do ED's exactly like Mathers, but I think it is worth understanding when you are choosing to do something differently.
Mary