1st: 5 packs of current set
2nd: 3 packs of current set
At the start of a Booster Draft, each player opens a booster pack and picks the card he or she wants from it. (You can't see the cards that the other players draft.) Then each player passes the rest of the pack to his or her left. You pick up the pack that was passed to you, select a card, and pass the rest to your left. This process continues until all the cards have been drafted. Next, each player opens a second pack, but this time, you pass the pack to your right. After all those cards are drafted, you open the third pack and pass to the left again.
Here's a diagram of the process of passing the packs:
Pack 1 Pack 2 Pack 3
Once all the packs have been drafted, players build decks from the cards they selected, adding as many basic lands (Plains, Islands, Swamps, Mountains, and Forests) to their decks as they like. Each deck must have at least 40 cards.
Each pair of players then plays a match, which consists of the best two out of three games. In the next round, the winners play against other winners (and the losers against the losers, if you like). The player who wins all three rounds—or all three matches—is the winner. It's a great way to spend a few hours with your buddies.