Westerado: Double Barreled

On the back wall inside the Sheriff's Office are wanted posters. They can also be found on the outside of the Sheriffs's Office and Saloon in Clintville and the Saloon in Santa Anna. These will be one of three types. "Wanted Alive," "Wanted Dead or Alive,' and "Wanted Dead or Deader." The easiest method is to select only the ones where Dead is an option.

Select a poster and you will see an image of the wanted man and his location. The following are the locations I encountered:

The Forest: These are the screens on the main map that are green. Look in the North center of the map and the area South of the Indian Camp
The Desert: This is the light colored area East of Santa Anna
The Mountains: The dark colored are between Clintville and the desert.
Around Clintville: Includes all of the adjacent screens to Clintville.
Around Santa Anna: Includes all of the adjacent screens to Santa Anna.

TheOnlyMatto commented that he encountered this location:
"Around the Train Station" and found him at Clintville Station

Go to these areas and look for a guy matching the description. The image is added to your journal and the location is listed with your quests. Kill him quickly so as not to make locals hostile to you and the Undertaker will immediately show up and pay you. You can do this whenever you want, but it is also a mission given to you when going through The Sheriff's quest line.
After you do five, the achievement unlocks.