Having succeed in another mission for the Stormcloaks, it is time for you to report back to Galmar Stone-Fist. Head to the Hjaalmarch Stormcloak Camp, which is now marked on your map. Talking with Galmor and he will tell you about the Imperial Courier
Find the Imperial Courier
Head to either Dragon Bridge or Rorikstead. In Dragon Bridge, you will want to head to Four Shields Inn and you want to go to Frostfruit Inn. Inside either, talk with the Innkeeper. They can be persuaded, intimidated, or bribed.
Retrieve the Imperial Courier's package
They will tell you that the Courier has just left but if you wait, they will come back. If you wait an hour or so, the Courier will come back into the Inn.
You can also attempt to catch them on the road. In ways, this helps a bit more because it is away from people. The Imperial Courier will be found halfway between the two locations of Rorikstead and Dragon Bridge.
Either way, loot the body for at least the Imperial Documents.
Bring the documents to Galmar Stone-Fist
With the Imperial Documents, time to head back to the Hjaalmarch Stormcloak Camp. Report back to Galmar Stone-Fist. He will have some comments to make on the information in the documents. After that, he will tell you that you need to deliver these documents to the appropriate person.
Bring the forged documents to Legate Taurus Duilis
This will complete the quest.