Poker probability question

I've read the wiki page, visited the Wizard's other website, and visited some other poker sites and .edu sites. I've grasped why the odds of drawing a pair or a royal flush and some of the in-between hands are what they are; i.e., I'm improving my understanding of poker probability. I've also found many tables telling me the odds of drawing various cards to complete various hands. And I've seen many pages, including the Wizard's, laying out strategic approaches to video poker.

But I don't know how exactly to calculate odds of drawing hands when ceertain cards are held. I'm a "trust and verify" kind of guy. I've looked at textbooks, but haven't been able to cut to the need-to-know material.

Where do I learn the math behind calculating video poker odds of drawing different cards or card combinations when holding a combination of cards, and/or better understanding poker probability?