I created about half a dozen of these with various reproduction wanted posters. The posters were already distressed by the manufacturer and aged using a special technique. A similar process would be to soak the poster in tea or coffee, then dried, and after application, coated with a clear finish.
I mounted the wanted posters on backer board and cut wood frames. The frames were distressed and then stained--I like to use Minwax Early American which is, I think, much like a light walnut color On some of the other framed posters, I included embellishments such as a rope, sheriff's badge, old replica coins, and stage bullets.