Leaders and Co-leaders can take their Clan to War by opening the Clan War screen and pressing the “Start War” button. The Leader who initiates the war selects the players for the war lineup from all the players who have their "Clan Wars" button green in their player profile. Each player can use the button to indicate if they are available for Wars or not.
This will start the search for an opposing clan. The matchmaking system will ensure your Clan is paired up with a Clan of roughly equal Strength (both attack and defense) and the same number of Clan members participating in the war. The Clan War will start immediately after a suitable opponent has been found.
Finding an opposing Clan can take several minutes, but there is no need to wait in the matchmaking screen. Just press the 'Return Home' button to continue playing, and you will be notified when an opponent has been found.