In my limited experience, cowpunchers and ranch hands speak directly and bluntly. They say exactly what they mean. It's the opposite of Japanese politeness-through-vagueness. Here are a few useful terms
Mosey: walk
Reckon: think, as in "I reckon I should apologize to that there preacher."
Yellow: cowardly (adjective)
Hankering: a desire, as in "I got a hankerin' for beans."
Beware that if you use all of these and also shorten words like "fella" and "hankerin," your dialogue might come off as exaggerated and cheesy.