This fun quest can be found in the village of Lurtch located in Velen. You can see where on the map below. To start the quest simply approach the bandits who are sat around a fire and speak to them.
The recommended level for this quest is 10. Most of the enemies are between level 5 and 6 so most people who have already finished the game will not find it challenging. The dialogue and story of the quest are excellent and worth experiencing.
You have to manually download this DLC is you are on Xbox or PS4. If you have The Witcher 3 on Steam then it should automatically patch for you.