You get basic lands separately, so really you're drafting 22-24 cards from the booster packs. Stores will typically have a big box full of basic lands, and after you finish drafting, you get the relevant ones from there (make sure to return them afterward).
To go into more detail about the draft, each player has three booster packs. Each player opens one, removes any basic lands and tokens/tips (that player keeps them), selects a card, then passes to the next player. This repeats until there are no more cards to pass, whereupon everyone opens a new booster pack. You repeat the selection and passing, except you pass to the player on the other side of the last round (so if you were passing to the right, you now pass to the left). You do this with the third booster pack too, except you're passing back in the direction of the first round (so right, then left, then right).