You can begin this quest by speaking to Cliffwatcher Longhorn in Freewind Post, Thousand Needles. He says:

My brother Nataka is stationed at Ghost Walker Post over in Desolace. I'd like to ask you... could you give this report to him for me? A year has passed since the last time I wrote, there has been unusual centaur activity since then and he should be informed.

Journey to Stonetalon Mountain north of the Crossroads, and then head west into Charred Vale. From there, head south into Desolace; there you will find a road going south to Ghost Walker Post.

Travel to Desolace
Find Ghost Walker Post
Speak to Nataka Longhorn

Don?t listen to his directions about going through Stonetalon if you ask me. Instead go west through Thousand Needles and follow the road northwest through the jungles of Feralas. The road may not be shorter, but it will take you through a safer route allowing you to stick to the road for the entire trip. Once you reach Desolace, continue north to the crossroad and take the east road. Make your way through Mannoroc Coven - watch out here - and you will find Ghost Walker Post shortly after. Speak to Nataka Longhorn who will say:

I know not what connection you have with my brother but it would seem he has trust with you.

I am Nataka Longhorn and would like to welcome you to Ghost Walker Post, we have many needs of an ally! Talk with others, perhaps there is need of you here.

Now you?re done, but before you leave, you might want to begin the Regthar Deathgate quest in Orgrimmar, since this quest line will eventually lead you here. If you?re up for some more running you could also take a trip back west to Shadowprey Village where you will get quests like Hunting in Stranglethorn among others.