I usually teach people the game three times in a row. Just, it goes so fast. It starts out, "This is a game of building a deck of cards. You have your own deck, your own discard pile, a hand, a play area. The deck is face down, the discard pile face up." You will be saying that three or four more times as people put their discard piles face down anyway. "We take turns. On your turn first you can play one Action card from your hand, doing what it says; then you can play any number of Treasures and buy a card for however much money you have; then you take everything you played and everything you didn't and discard them all, and draw a new hand of 5, shuffling as needed." Well that's not enough so back over it again. "First you can play one Action card. They say Action on the bottom. You follow the instructions on it. If there's a dividing line you stop there, stuff after the line happens at some other time it will say." And so on. After the slightly longer version of the rules, it's back to explain the terms: the four +'s, gain, trash. "So if you played Militia you'd be done, but you could play a Market and a Market and a Market and then a Militia, because they each gave you +1 Action." And so on. Then, man, what were the rules again? It takes no time to repeat. "We take turns. On your turn first you can play an Action card..." Now maybe I mention ABC. A key thing is that buying cards doesn't do anything (except when it does), it just puts them into your discard pile; after you shuffle you'll start to draw cards you actually picked out, and will be doing those things. I explain that the money is income; some people expect to lose a treasure when they buy something, like in some game that made more sense.
I am teaching somebody who showed up to play, but I am playtesting and it's not the main set. So then I explain whatever new stuff is going on.
I do give some strategy advice. "Your initial ten cards are bad." "If you had a hand of 5 Silvers, that would make $10. A hand of 5 Coppers makes $5. One way to get from the $5 hand to the $10 hand is to buy better cards... another is to get rid of weaker cards." "If this weren't your first game, you'd be trying to figure out how things would go this game, given the cards dealt out, what strategy to pursue. But you aren't winning anyway, so just buy some stuff to try it out and you'll see how the game works."
Then of course I am there during the game, so when they play a Lab directly to their discard pile or something, I can say, no dude, into play. And they'll watch our turns too.