After you first arrive at the Golden Apple Archipelago in Genshin Impact, you will be able to explore the area and start a new quest. As long as the three devices have been activated and the fog has been cleared, you will be able to do the Main Cannon, Make Ready, Fire! portion of the Midsummer Island Adventure event.
This involves going around the map fighting groups of Hilichurls. To earn rewards, you will need to complete certain challenges, including collecting Shiny Flotsam.
Flotsam can be earned by sailing to Hilichurl platforms and clearing out the enemies that can be found there. These platforms will be marked on your map by small Hilichurl icons.
When you arrive, you will find numerous enemies on small and large platforms. The smaller platforms can be destroyed by your ship, but you will need to exit the craft and jump up to the bigger platforms to take out the enemies that are there.
Clearing the platforms out completely will earn you 200 Shiny Flotsam which will count towards your daily challenges for the event. You will not receive the 200 Shiny Flotsam until the last enemy is dead, so make sure you clear them all out.
You should also be sure to destroy as many platforms, crates, or barricades as you can, as other challenges will require you to do this so it’s more efficient to work on everything at the same time.