Speak with Quartermaster Kaoshin at the Black Tooth Hovel in Burning Steppes.
This isn't really the most... convincing disguise I've seen, but it'll work. Undercover work is all about swagger, anyhow. Just act like you're supposed to be there and you'll be fine.
Your contact inside Black Tooth Hovel will be an orc by the name of Quartermaster Kaoshin. He does NOT know you're a spy, so try and stay cool.
Do whatever Kaoshin asks you. Colonel Troteman will be in touch via gnomecorder to give you any special instructions.
Good luck out there, soldier.
You will receive: 35
Where have you been, grunt? You're late! Give me one reason why I shouldn't kill you on the spot!
- Colonel Troteman says: Don't forget to put on that disguise!
Exit and turn due south. Kaoshin is standing outside the Pillar of Ash just across the road. So long as you are wearing the [Blackrock Disguise], he will be friendly.
Once you put on your disguise, you discover the reason for Keeshan's concern: it's nothing more than altering your sleeves, removing any tabards and a Hallow's End mask!
Optional breadcrumbs:  Mouton Flamestar or  Hero's Call: Burning Steppes! or  Warchief's Command: Burning Steppes!
-  Burning Vengeance
-  Stocking Up /  A Future Project /  Mud Hunter
-  The Sand, the Cider, and the Orb
-  Warlocks Have the Neatest Stuff /  Shadow Boxing
-  Return to Keeshan /  Return to Ariok
-  Chiselgrip, the Heart of the Steppes
-  A Needle in a Hellhole
-  Prove Yer Allegiance /  Scrapped Golems
-  Golem Training
-  A Perfect Costume
-  Into the Black Tooth Hovel
-  Grunt Work
-  The Kodocaller's Horn
-  Taking the Horn For Ourselves
-  General Thorg'izog
-  Trial by Magma
-  I Am the Law and I Am the Lash
-  Enough Damage For One Day
-  Morgan's Vigil /  Flame Crest
-  Blackened Ashes /  Latent Demons of the Land /  A Heap of Delicious Worg
-  Blood Tour /  Draconic Vanguard /  A Deal With a Dragon
-  Placing the Pawns
-  A Delivery for Thorg'izog /  A Delivery for Neeralak /  A Delivery for Xi'lun
-  Locked and Loaded
-  Assault on Dreadmaul Rock
-  Glory Amidst Chaos
-  The Spoils of War
Optional breadcrumb to Swamp of Sorrows:  The Bogpaddle Bullet