When the time slot is active (00:00 UTC, 06:00 UTC, 12:00 UTC, 18:00 UTC), you have 10 minutes (until 00:10 UTC for the first one, for example) to click the button to join a time slot. You should see it on https://www.warzone.com/Clans/War
. From there you can pick a template to join on.
The games start at :10 so you can join anytime between :00 and then.
Edited 3/16/2021 20:38:58