You keep playing each trainer until you get four stars on each one.
For each four stars you get a non-tradeable booster pack.
If you complete a league you get some bonus non-tradeable booster packs.
To easily do this, you play on Expert difficulty.
Be aware there is currently a glitch where it resets the difficulty & opponent after each game.
In order to bypass that, choose the league and change the difficulty to a lower difficulty & then select the difficulty you want.
Then choose which opponent you want to face.
It's best to just play the Basic Black/White decks against them.
Unless you have a really good starter deck unlocked.
It usually takes about 6-10 games (or so) to get the four stars.
Depending on how badly you beat them or they beat you.