- Get your orders from Galmar.
- Locate the Courier at Dragon Bridge Inn or the Rorikstead inn.
- Retrieve Imperial Documents from him
- Return them to Galmar Stone-Fist
- Take the forged documents to Legate Taurinus Duilis in Morthal
- Return to Galmar Stone-Fist
Galmar Stone-Fist will tell you that you need to deliver some forged orders to a imperial legate in Morthal but first you have to find some orders to be forged. He tells you that Dragon Bridge and Rorikstead are hotspots for imperial couriers, and you should look in one of those places.
Head to Rorikstead or Dragon Bridge and go into the Inn, speak to the innkeeper. To find out his location, you have to pass a bribe, intimidate or speech check, the innkeeper will say that he just left, so you could go after him, or you can wait a while for him to come back. It is better to find him in the wilderness because killing him, even if he attacks you first, will put a bounty on you. Although he may be a far way out of town so you could wait for him to come back and then follow him out of the town. Once you find him, telling him you want the documents will make him attack you immediately, so you can either kill him or pickpocket the documents.
Once you have the documents, head back to the Stormcloak camp and give them to Galmar. He will then forge the documents and tell you to take them to the Legate in Morthal. Head to Morthal and enter the Highmoon Hall, the legates through the doorway under the stairs to the left. Talk to him and give him the documents, if you are not wearing a Legion uniform he will ask about it, but he will take the documents and give you five gold. This will start the next stage in Liberation of Skyrim.