Have them play a few games with the base set where they have no idea what the hell they're doing and just grabbing whatever. This will cement mechanics and terminology in their head. You can't really explain anything at all if they're not solid on basic mechanics, like starting hand size and the fact that you only get 1 action per turn. They'll also run into a few common scenarios, like colliding terminals without enough actions, or dead draw, or how hard it is to get $6 to buy a gold, or drawing $7 in the endgame. They'll probably be building a piecemeal village-idiot deck at first. That's normal.
Most newbies overvalue kingdom cards and undervalue money. Teach them the importance of money by kicking their ass with a dead-simple BMU deck. Don't mention BMU specifically or how it stacks up against other deck types, just drive the point home that if you want to buy things you need money so you can afford them.
Then kick their ass once with a simple, no-frills chapel-based deck. After they're done scratching their head, explain the concept of density. This doesn't need to be deep or mathematical, just simple like "you only get 5 cards a turn to start, so you want them to be as good as possible. Copper and estates are the worst cards in your deck, so removing them makes it easier to get the good cards. I mean c'mon, did you ever draw a card and be glad it was a copper? Not often."
Once you've done this, they'll have enough of the fundamentals that you can explain other stuff to them. Deck density relates to things like terminal collision and when you want to start greening; average card value relates to the importance of hitting the magic thresholds of $5, $6, and $8. The other major concepts to introduce are attacks, and 3-pile endings. The former is probably best done with Witch or Militia, the latter is probably best done with a Gardens kingdom. After that, they'll need experience experience experience.
