Instructions
Travel to the town of Rathskeller Fork. It is at the northernmost point of the region labeled "Gaptooth Ridge" on the map. You need to find a Treasure Hunter bounty poster. While they can be found in various places throughout the game, you are most likely to find one in Rathskeller Fork.
Look for a wall made of vertical wooden slats in Rathskeller Fork. The wall is northwest of the town entrance, on the far side of the open central area of the town. This is the location where you are most likely to find bounty posters, but their locations and placements are randomized. If you don't find any posters there, leave the town and come back in a few in-game hours.
Read all bounty posters that you find in Rathskeller Fork. The Treasure Hunter Gang is one of several. You need to find a bounty poster that specifies the target as a member of the Treasure Hunter Gang. Each poster features a picture of the bounty, his gang affiliation, his name and a brief description. Once you find a Treasure Hunter Gang bounty poster, pick it up after reading it to begin the bounty hunting mission.
Open the in-game mini-map. Look for a red circle icon on the map. This icon marks the location of the Treasure Hunter bounty you have to capture. Go to this location, and be prepared to fight.