Follow the western road from Razor Hill to the Barrens over a bridge. Stop at the orc outpost across the bridge. Give Kargal Battlescar at the Barrens outpost your recruitment letter.
Hmm, you look pretty strong.
Listen, my good friend Kargal needs some new recruits for Barrens duty. I know you want to do what's right for the Horde. Well, this is your chance.
Take this recruitment letter to Kargal and see if he'll sign it.