New Main Objective: Blackmail Raerek
Look around the Understone Keep's main hall for Raerek (the above screen). If he's nowhere to be found, wait until the next day, otherwise you'd have to risk going to the closed area again. Before meeting Raerek you might also be held off by Faleen, but no matter what you tell her, you'll be able to brush her off. Talk to Raerek and tell him about the Amulet of Talos, and he'll ask you to follow him to a back room.
Follow Raerek and stay close to him as advised (the above screen). Your second conversation with him takes place once you get to his bedroom, from which you stole the amulet earlier.
After you reveal to him that you are a member of the Stormcloak Rebellion, Raerek offers that he keeps the information of him worshipping the forbidden god in exchange for information on a shipment of silver and weapons. If your Speech is high enough, you can try to persuade him into giving you additional gold pieces (the above screen) and agree to the agreement only then. Raerek mentions a wagon on its way from Markarth to Solitude. Because it doesn't travel fast, you have a lot of time to catch up to it, attack and rob it.
New Main Objective: Report to Galmar Stone-Fist
Leave the Understone Keep, open the world map and return to the Reach Stormcloak Camp. Once there, locate Galmar Stone-Fist and tell him the news about the caravan (the above screen). He suggests meeting the Stormcloak scouts present in the area and organizing a joint attack on the caravan.
New Main Objective: Meet the Men
When you open the world map you should see that the scouting party is stationed on the road west of the Stormcloak camp (the above screen). It's best not to use Fast Travel at all, but journey there on foot or by horse. Be careful while passing Broken Tower Redoubt, because you can be attacked by renegades there.
When you arrive at your destination, look around for the scouts (the above screen) and talk to Ralof, who is one of them. He tells you that his party has been tracking the caravan for a time now, but didn't know what it was carrying until now. At the end of the conversation, Ralof leaves you with a choice. You can either attack the caravan by yourself or be a part of a trap the scouts are planning to set up.
New Main Objective: Follow Ralof and ambush enemy scout
New Main Objective: Take over the caravan
If you want to act alone, go southeast right away. You can attack the caravan guards from a distance, but because of the fact that the majority of them are archers, it's safer to fight them in close combat (the above screen). If by any reasons you're starting to loose, go back to the scouts quickly so that they could join the battle.
If you agreed to Ralof's plan, wait for the night to fall in. Crouching, follow the scouts. Try to stay close to Ralof. Stop when he stops (the above screen) and wait until the gives orders to quietly eliminate the enemy scout.
Only then move forward to the Imperial Legion's camp (the above screen). With any luck, you will surprise the enemy soldiers, and besides you have your allies to help you. Fight until every caravan guard is dead.
New Main Objective: Report to Ralof
Before you get back to Ralof, examine the enemy camp. Try to open one of the chests on the wagon, as it holds a few valuable items and taking them won't be considered stealing from the Stormcloaks. Approach Ralof and listen to a short commentary (the above screen). This mission is now over, so you can return to the Liberation of Skyrim (main objective: Liberate the Reach).