First go to NPC JinxJinx View on map - you have to go there with a party of 4 people.

You will have to choose between three stages: easy, medium and hard. In this tutorial we used a easy stage as an example.

Player: hi
Jinx: Hello Player. Do you want to do dungeon?
Player: yes
Jinx: Which difficulty level do you choose: easy, medium or hard?
Player: easy
Jinx: You have chosen an easy level. Are you sure of your choice?
Player: yes

You with your all party members will be teleported to an Dungeons island on which you'll have to kill group of monsters. If all monsters are killed then a boss will spawn on position of last monster has been killed. After you kill a boss you'll be teleported out to get a reward for participating.