Wonderland is a Raid dungeon that opens only through a collective effort across all players to donate different items to different "NPCs".
The first stage is donating Red Jewels from Jawaii Dailies to the Poring Rock on top of a cliff. If the rock has a red aura, this stage has been completed.
The second stage is donating Blue Jewels and Transparent Mushrooms looted from monsters in Jawaii's beach to the Nam nam Statue in the village. If you can see sparkling blue jewels on the statue, this stage has been completed.
The third stage is donating Fluffy Mushrooms and Sharp White Flowers looted from forest monsters in Jawaii, to the hidden Mr. Rabbit at top of Jawaii Love Island. The only way to know if this stage was completed is talking to him, or checking the 4th stage NPC.
The last stage requires you to donate Luminous White Flowers to the stone slate by Wonderland's entrance. If you can see the portal open, this stage was completed. I'm not sure where these are obtained.
If someone completes a stage while you're in Jawaii, you'll see an announcement on your screen.