Starting the mission
Reach Valentine town – there, you will be able to start this mission. Arthur can reach this place shortly after starting the second chapter. A Stranger’s question mark will appear in one of the saloons. Speak with the people sitting at the bar.
You get 4 photos of ex-gunslingers. Open your satchel and examine all pictures. Remember to flip every single one of them (by pressing the right button). This gives you locations of all the gunslingers. You can reach them in any order.