Find the Gremlin's Checkered Flag Check-In under the Event menu at the bottom-right of the game screen during the event period.
Event progress will be shared across all characters within your account.
Reach the daily cumulative login time of 1 hour with character(s) at Lv. 50+ during the event, to receive the reward for your relative cumulative check-in count in the Account tab of the mailbox. Rewards can be claimed at the Account tab of the mailbox by logging in with your chosen character.
If you miss a login day, you can make up for the missed check-in by using an item called "Gremlin's Last-Minute Chance."