After reporting to Galmar Stone-Fist you will be told that Raerek, the steward of Markarth, is secretly a worshipper of Talos and that you will need to find some evidence of this, and then convince him to help the Stormcloaks. Head to Markarth and go into the Understone Keep, find Raerek’s quarters and in a cabinet against the wall, directly in sight from the doorway, you will find a inscribed Amulet of Talos.
Show the amulet to Raerek and he will tell you to follow him, he will lead you to his quarters. When you get in his quarters he will immediately begin speaking, assuming you want to extort something from him. Asking him if you could come to some sort of agreement will have him mention a shipment of silver and weapons, selecting any topic except “All right, it’s a deal” will give you the chance to use speech to gain a leveled amount of gold. After he tells you where the wagon is, he will take his amulet back and you will have to return to Galmar.
Return to Galmar Stone-Fist at the reach camp and, after telling him that there is a shipment of coin heading to Solitude, he will tell you to meet with some of his scouts that are situated along the road and ambush the wagon. Follow your quest marker to the scouts, once you find them, speak with their leader Ralof. He will inform you that the wagon had an accident, and is stranded up the road, he will then tell you that he and his men will eliminate the sentry on top of the hill, and then you should go in and distract the imperials so that they can shoot the imperials with their bows. You can also tell them to stay there while you just go in and kill them all.
If you decide to go with Ralof’s plan, tell him you’re ready and then creep along with him and his men until he stops at a rock. He will then command his men to fire, and they will kill the guard on the ledge and take up positions along it. Now you have to head down to the Imperials. When you approach them, they will pull out their swords and tell you to move along, just simply attack them and Ralof’s men will begin shooting the Imperials. Kill all the Imperials and talk to Ralof. That will complete the quest and you will have to head back to Galmar.