New Main Objective: Find the Stormcloak Courier
Note! You get this mission during the Reunification of Skyrim main Imperial Legion Quest and its completion is required to advance to the next part.
According to Rikke's tip, you can find the courier in one of two inns. The first one is The Nightgate Inn (the above screen), but it's harder to get to because it's not located in any town. If you choose is nevertheless, start you journey from Korvanjund ruins and go northeast. The inn is situated on a hill.
The second one is The Candlehearth Inn (the above screen), and here the case is simpler because it's located in Windhelm. What's more, you don't have to worry that by going to the Stormcloaks main city you'll be attacked; the Stormcloaks will limit themselves to saying that you're not welcome here.
It doesn't matter which inn you choose, because in both cases the storyline is the same. Approach the innkeeper (Hadring in The Nightgate Inn or Elda Early-Dawn in The Candleherath Inn) and ask for the Stormcloak Courier (the above screen).
The innkeeper refuses to share any details on their clients, leaving you with a few options. The best one is to use highly increased Speech to try to persuade the innkeeper to tell you want to know (the above screen) or to intimidate them. If you fail, you can also bribe them or simply wait for the courier to arrive at the inn.
New Main Objective: Retrieve the Stormcloak's Courier package
If you managed to persuade one of the innkeepers to tell you about the courier, his current location will be added to your map. It's very probable that he's is on the way between the two inns, and if so, you can meet him there. When you find the courier (the above screen), you can talk to him or attack him. The main advantage of this method is that you won't be spotted and attacked by any third party. After the fight, examine the courier's body and take the Stormcloak Documents.
If you failed to persuade either of the innkeepers to tell you anything the courier, or you simply want to avoid bloodshed, you can stay at the inn, fast forward time and wait for the Stormcloak Courier to arrive (the above screen).
When he arrives, wait until he goes to rest (the above screen). This way you can easily rob him or kill him in his sleep, leaving him no options to fight back. In the latter case, make sure your actions aren't noticed by the innkeeper or other patrons. See if the Stormcloaks Documents have been added to your Inventory.
New Main Objective: Bring the documents to Legate Rikke
Open the world map and return to where Legate Rikke is, the Pale Imperial Camp. Find Legate Rikke and give her the courier's documents (the above screen). She edits their contents and sends you to the Stormcloak Commander in Dawnstar with the forged documents.
New Main Objective: Bring the forged documents to Frokmar Banner-Torn
Open the world map and go to Dawnstar, located east of the Imperial Legion's camp. Depending on the time of day, Frokmar Banner-Torn could be in one of the buildings or walking through Dawnstar (the above screen). Initiate dialogue during which you will automatically give him the documents.
Note! You can now return to the Reunification of Skyrim (main objective: Regain Pale).