You'll be able to accept this Stranger Mission from Sampson Black and Wendell White, after you've completed the The Joys of Civilization Main Mission in Chapter 4.
Once you've completed that Main Mission, head over to the south of Rhoades where you find Sampson Black and Wendell White having a fight near a tree, intervene to begin the mission. After speaking with the guys they'll ask you to head into the town and take down five posters that have been scattered about.
You'll be able to find the first poster leaning against a wooden pole with a light in the center of town which may require you to fight a guy to get him off the pole to grab the poster. The second poster can be found on the Shreiff's notice board, and the third can be located in the Post Office.
The fourth poster can be found on another wooden pole at the end of the road, and the fifth will require you to force a guy off his horse and attack him to get it.
Once you've picked up all five posters, head back to Sampson Black and Wendell White where you'll face two options which will affect how this mission plays out.
- Option #1 - Help Sampson Black and Wendell White
When your arrive to see Sampson and Wendell, simply burn the posters to continue there storyline and meet up with them again at a later point in the game.
- Option #2 - Turn Sampson Black and Wendell White in
When your arrive to see Sampson and Wendell, you can choose to take out you hogtie and capture both of them. Once you've got both of the guys tied up, place them up on your horse and deliver them to the Sherrif in Rhodes which will complete the mission.
Part III / IV
If you chose the second option and decided to help out Sampson Black and Wendell White, you'll be able to find them towards the north of Rhodes, but unfortunately they're pretty unwell so they'll need you to give them any medicine that you may have or if you don't have any on you, head back into town and purchase some and deliver it.
You'll need to visit the northern point of Big Valley to see Sampson Black and Wendell White one last time to complete this mission.