How do you get booster packs?

You get them randomly once you got all your card drops for that game. Get all your card drops ( for a game ), then hope for a booster pack(for that game).

If you have used all the drops for a game, just login to Steam each week.

Basically if someone crafted a badge (for the same game), then someone who has no drops remaining will be randomly chosen to receive a booster pack.

Read

wait for it to drop into your inventory

just wait and the magic will happen!