Reach Adventure Rank 20 or above. And complete Prologue: Act I "The Outlander Who Caught the Wind."
How to start the event
Start by talking to the "Former Friend of Seelie" NPC, Ulman, in Stone Gate area. He will give you a treasure book with a map for each treasure locations. Ulman will mark out two new areas in the book each day, for a total of 14 treasure areas.
This event rewards offers 300 Primogems, 80 Hero's Wit, 80 Mystic Enhancement Ores, 200,000 Mora, and one of the three available mini seelies to keep, outside of the event. Total amount of Iron Coins needed to buy them all is 840 (1 seelie only).
Finding the treasures
Check the book for a hint to the map location, by tapping on the "Event Details" button when you view event information (F5 on PC, or the compass icon on the upper right side of your screen, next to the battle pass icon).
Head to the location shown on the map to find places that you can interact with. Each spot that you dig will always get you some iron coins, sometimes it can also give other items.
You can equip the Treasure-Seeking Seelie from your inventory to help guide you to the places that you can dig, but it can only be used when you are in the treasure area. When you get close to the treasure spots, the Seelie will start to shine brightly, and start to make a noise. Turning on the seelie in the treasure area will also show you the radius of the treasures on your map.
Equipping it isn't necessary, since you can still interact with the digging spots without the seelie. If you need the gadget spot for other equipment, like the NRE, then you don't have to equip it.
After every few locations, there are special challenge areas to get some more treasures. These challenges can be done in Co-op, or solo. You can get rewards from the Co-op challenge from other players' worlds up to 3 times, throughout the event duration. Once the event ends, you will not be able to take part in these co-op challenges.
Treasure Area 14
The last treasure area is on the top left island in Liyue Region, next to Qingce Village, or north of Noblesse Oblige domain.
Treasure Area 13
Treasure area 13 is in Guyun Stone Forest, which is east of Liyue Harbor. The middle island has 4 Ruin Guards, but you don't need to clear them in order to get 60 iron coins. You can circle around the island and go to the island on the left to get 60.
Side note: there are also ore deposits on the small island to the bottom right of the circle.
Treasure Area 12 - Co-op trial challenge
Back to Liyue region, treasure area 12 is in Tianqiu Valley. This is west of the Pyro Regisvine boss. There is also a co-op trial challenge not far west of the teleport waypoint.
Treasure Area 11
This spot is near the Anemo domain (Valley of Remembrance), in between Mondstadt and Dragonspine.
Treasure Area 10
Back to Mondstadt region, treasure area 10 is in Cape Oath, which is just west of spiral abyss island (Musk Reef).
Treasure Area 9
Treasure area 9 is below Mt. Hulao, northwest of Liyue Harbor.
Treasure Area 8 - Co-op trial challenge
Treasure area 8 is in the upper part of Stormterror's Lair.
There is another Co-op trial challenge, just below the stairs, near the teleport waypoint. The trial challenge will appear once you've collected the iron coins in the area.
Special Treasure 2
The second special treasure 2 location is south of Yaoguang Shoal, which is just south of Mingyun Village (treasure area 7).
Treasure Area 7
Treasure area 7 is in Mingyun Village. North of Liyue Harbor, or southwest of Dragonspine.
Treasure Area 6 - Co-op trial challenge
In Windrise region, east of Mondstadt.
After collecting the iron coins in the area, a new tutorial will pop up, and a co-op trial challenge will appear. Throughout the event, you can get rewards from other players' world up to 3 times. Be sure to mark them on your map so to remember the location, and do them to get additional rewards before the event ends.
Treasure Area 5
Treasure area 5 is in Cuijue Slope. In Liyue region, north of Liyue Harbor.
The special treasure location is to the west of the Brightcrown Canyon teleport waypoint.
Treasure Area 4
Treasure area 4 is in Guili Plains, which is north of Liyue Harbor.
Treasure Area 3
Third treasure area is north east of Mondstadt city, in Starfell Lake.
Treasure Area 2
Second area is in Lingju Pass, west of Liyue city.