You have to go to Clan Wars on the app and opt in. You'll have to pick your time slot at that time. I'm not sure exactly how early before the war starts that the option comes available but I know that you have to be opted in before the Bronze through Platinum war (probably 24 hours before) starts because otherwise they'll assume that you intend to compete in the Bronze through Platinum war and match you accordingly.
PS: Congrats on making it to Diamond! The competition in Diamond is pretty fierce but I have to say that being able to cut my war time down to a mere 12 hours has been the best part of it for me.