When Clan War Leagues is about to start, the Clan Wars button turns golden to indicate the arrival of Clan War Leagues.
The Clan Leader and Co-leaders have two days to enter War Leagues with at least 15 (maximum 50) players. The selected players form the pool of players that can be selected to fight on Battle Days. It's a good idea to enroll more than the minimum 15 people into Clan War Leagues to have more players who can do attacks in Wars during the Season.
Each Season Clans can choose from two war sizes: 15vs15 and 30vs30. Clans choose the war size when deciding the Roster for the Season. In Champion league, only the war size 15vs15 is available.
Clans can make the decision about war size each Season, and it doesn't affect their league position in any way. Clans can fight in the same league with either war size. If Clans switch between war sizes between seasons, they will continue from the league they reached regardless of which war size they played previously.
Each Clan member can indicate their ability to play in Clan War Leagues with the opt-in/opt-out button in their player profile. Clan Leaders can however enroll players to War Leagues even if they have opted out of wars.
Once a Clan has signed-up for Clan War Leagues, they can't play regular Clan Wars. If a Clan doesn't sign-up for War Leagues, regular Clan Wars are available once the sign-up period has ended.
What if a player is kicked out or leaves the Clan?
If a player is signed-up, but then leaves or is kicked out of the Clan, he can't play Clan War Leagues in another Clan until the Season is over. He can only play regular Clan Wars in another Clan. He can return to his original Clan to continue playing in Clan War Leagues at any time.
In terms of rewards such a player is still eligible for:
20% to 100% of the placement League Medal reward depending on how many War Stars he collected
Bonus League Medals given by Clan Leader and Co-Leaders