Tl;dr: pick your personal favorite doll, collect daily Penlights, fight the boss starting on December 11th.
The Glittering Starwish Event (Doll popularity contest) has 3 phases:
Initial phase (Dec 6th - Dec 11th) where you choose the doll you want to support and do daily tasks to collect Penlights (max 130 per day).
You can still change your preferred Doll during this phase (if you are indecisive, please note that there is a 6-hour cooldown between changes, so don't leave it until the last day);
World boss phase (Dec 11th - until the boss is dead) where you have to fight the boss (Target Dummy) multiple times per day (can use sweep based on your best attempt) and earn Penlights based on your score.
Final phase where the results are tallied up and the winner is announced.
The 3 most popular dolls get skins designed for them/their weapons. As for the personal rewards (the ones YOU are getting), it depends on your personal score (the amount of Penlights) on the leaderboard of the Doll you picked to support. Doll leaderboards are separated from each other. You can check the specific rewards in the event rules.
The World Boss has to be fought with your best team in order to get a higher score.
It has daily rotating buffs which are quite strong, so having dedicated elemental teams is advised. Though theoretically you could ignore them if your off-element team still scores more than e.g. Ice or Burn.
You get 3 free attempts per day and up to 27 paid ones. The cost of using up all 27 is 1780 collapse pieces per day.
Therefore, unless you're a whale and your team is getting close to max amount of Penlights when fighting the boss, it may not be worth it to buy any paid attempts at all. Regardless, make sure you at least participate.
