If you want to join the clan war, the clan master needs to register for it first on Friday.
For registration, there are 2 modes: Declaration (12:00-16:00 server time) and Auto-Match (16:30-21:00 server time).
Declaration (12:00-16:00): After the clan master begins Declaration, the match result will come out in around 15 minutes.
Auto-Match (16:30-21:00): The clan master is required to send Auto-Match request during the period and the auto match result will come out at 21:00.
After matching successfully in either mode, clan members can sign up for clan war.
Please go to Clan – Info – Go – Clan War Officiant to click the icon of ‘i’ on the upper right to know the complete rules of the clan war.