Fortunately, the developers of the game have produced a quick run-down of how it all comes together over at the official Supercell forums. Here's the low-down, straight from the developers' mouths:
- To join a league, start by completing one multiplayer attack.
- At the end of this attack, if your trophy count is higher than 400, you will be automatically placed into an appropriate league level (based on the league level's trophy range).
- You will be promoted to a higher league or demoted to a lower league if your trophies reach certain limits (listed below) for your league.
While being part of a league, the following rules apply: