Unlike constructed formats, where players arrive with a carefully constructed deck ready to play, limited formats allow players to build a deck from new cards as part of the game.
At the start of booster draft, each player opens a booster pack and picks a single card. (Don’t show the other players what you pick!) Then everyone passes the rest of their pack to player on their left, each player then picks a card from the pack they just received before passing again. This process continues until all the cards in those packs have been drafted.
Then each player opens a second pack, but this time, you pass the pack to your right.
After all those cards are drafted, you do the same with the third pack, passing to the left again.
Once all the packs have been drafted, everyone builds a deck from the cards they have selected. You may add as many basic lands (Plains, Islands, Swamps, Mountains, and Forests) to your deck as you’d like, but each deck must have at least 40 cards.
The best part about booster drafting is that you get to keep the cards you draft!
More details can be found here.