About the decans: Mel writes quite a bit about them on her website, but I'm glad she left us something to do with these cards.
The IMHo best way to understand them is to study the associated majors (the zodiac major and three planetary majors) and minors (the three decan minors), for example for the Decans of Cancer card:
majors: Chariot (for Cancer) and Empress (Venus, 1st decan), Magician (Mercury, 2nd decan), and High Priestess (Moon, 3rd decan),
minors: 2, 3, 4 of Cups
You may also take along the High Priestess because the Moon rules Cancer.
By looking at the decan card, comparing it with all the cards that are incorporated in the decan card, you can see where each motif comes from.
It's something between the intuitive and the intellectual.
There is actually no need for Mel to explain more than that. After you have all the information about the cards that make up the decan card, the decan card itself becomes obvious. But nearly unsayable because they're so full with symbols, associations and relations.
I find this act of synthesis in the card and in my head when I look at the whole "map" of cards one of the most enlightening acts in my tarot studies ever. My old little neurons are jumping wildly.
Right now, I'm only studying the deck, not reading with it - I should buy an additional... NO NO NO I won't.
ETA: I added my study layout for Taurus/Hierophant. Added the court cards for the decans, too - although the decan cards are, far as I could see until now, built from the decan cards and zodiac major.