Compelling Tribute (Stormcloak)

Walkthrough
While speaking with Galmar Stone-Fist after the last successful mission, the commander tells the Dragonborn that the steward to the Jarl of the Reach, Raerek, secretly still worships Talos and is at least somewhat sympathetic to the Stormcloak cause. He asks the Dragonborn to find evidence of this in order to blackmail the steward.

Find evidence
After travelling to the Understone Keep in Markarth, the Dragonborn then searches Raerek's quarters for evidence of the steward's allegiance.

Blackmail Raerek
The Dragonborn then blackmail's Raerek in order to obtain information about a caravan transporting gold and weapons meant for the Imperial Legion and heading for Solitude.

Report to Galmar Stone-Fist
The Dragonborn then returns to Galmar and tells him about the caravan. The commander then sends the Dragonborn to meet some Stormcloaks and ambush the caravan, marking the route on the map.

Meet the men
The Dragonborn then sets out to meet the Stormcloaks, who are led by Ralof.

Follow Ralof and ambush enemy scout
The group then ambushes the Imperial scout and obtains the exact location of the caravan.

Take over the caravan
They then take over the cavavan and loot it.

Report to Ralof
Having looted the caravan, the news of the successful raid is them relayed to Ralof.

Report success to Galmar Stone-Fist
Finally, the Dragonborn returns to Galmar with the good news.