As this is a guild event, you will first need to join a guild to participate. If you are already in a guild, you will be taken directly to the event when you tap the event button. You don't have to spend any resources to take part in the event - it's free.
Completing missions will earn you stars, which will fill up the event's progress bar. The exact number of stars that you will receive for completing a particular mission is shown in the "Reward" field.
Please note that if you have already received some rewards in one guild, and then join another guild, then you will not receive the same rewards again.
The rewards located at the bottom of the progress bar are your personal rewards. They are locked by default and can only be accesses by purchasing a Battle Pass. This will allow you to receive both guild and personal rewards when you reach certain marks on the progress bar.
Please note that the pass will only be active for one launch of the event. You will need to buy it again for the next events.
Unlike guild rewards, if you change guilds and join a guild that is further ahead in the event, you will be able to collect all the personal rewards that the guild had achieved before you joined.