Our goal is to intercept an Imperial Courier to steal his documents, which Galmar plans on using to make forged documents for us to deliver to the enemy commander in Morthal. The easiest way to do this is to simply head over to the town of Dragon Bridge and go into the tavern. Here you can talk to the innkeeper and ask her about any Imperial Couriers she may have seen. Now you'll have the choice to PERSUADE, BRIBE, or INTIMIDATE her. Personally I tried to persuade and it failed (speech was at 46), so I just bribed her instead.
Now you'll have the location of the courier on the road! This makes it easy to go out there and find him. He'll be alone, so it's just as easy to take him out and take his documents.
Note: To TRUELY make this an undercover mission, take his clothes and equip them before reporting to the Legate.
Now bring the documents back to Galmar. He'll make a few corrections to them and send you off to give them to the Imperial Legate at Morthal. Head on over to Morthal and give the Legate the documents.
Note: You can equip the Imperial equipment to make this go smoothly, but even if you don't you can tell the Legate you're wearing what your wearing to make sneaking easier... and he'll buy it!
After he accepts the documents this mission will end. No other mission will start until you go back to Galmar and talk to him again, at which point he will state that we're going to invade and take over Fort Snowhawk, leading to our next mission: "The Battle for Fort Snowhawk".