A False Front (Stormcloak)

Walkthrough
Galmar Stone-Fist tells the Dragonborn that the Stormcloaks need to deliver some forged orders to Legate Taurinus Duilis in Morthal. In order to do this, the Imperial Documents need to be intercepted so they can be forged. He then says that the Imperial courier should be in either Dragon Bridge and Rorikstead.

Find the Imperial courier
It can take a few attempts before the courier is located. Speaking to the innkeeper in either town will provide some clues as to his location at the time.

The innkeeper at the Frostfruit Inn, Mralki, will require some stern talking to though, in form of persuasion, intimidation or 845, to reveal information about the courier.

Retrieve the Imperial courier's package
To retrieve the package of documents, the Imperial Documents, the courier can either be pickpocketed or killed and looted. If the Dragonborn opts to kill the courier, this should be done outside either of the towns and out of sight of anyone, otherwise a bounty will be incurred.

Bring the documents to Galmar Stone-Fist
Once the documents are secured, they must be brought back to Galmar Stone-fist and exchanged for forged copies, the Forged Documents.

Bring the forged documents to Legate Taurinus Duilis
The forged copies must then be brought to Legate Taurinus Duilis who is in Morthal. The Dragonborn, pretending to be a courier, receives a whopping 5 reward for the effort.

Die falsche Front (Sturmmäntel)