Now you’re sent to take Fort Greenwall. Fast travel to the nearest location and make your way to the marker. When you meet the soldiers, its time to take the Fort. This is pretty similar to the previous quest where you had to take the Fort. Go in there and start killing Stormcloaks over and over until the counter reaches 0%.
Take extreme caution not going alone, since they like to come out in hordes and they can kill you very easily. Stay with the Imperial troops and retreat a little if need be. You can also use the weapon you were given during the quests of the Civil War. This, at least in my case, helped me out a bunch taking care of the Stormcloaks.
Return to Tullius at Solitude and now we’re to go to the camp near Winterhold, all the way to the north side of Skyrim. Fast travel to Dawnstar and make your way to the camp. Once you arrive, speak to Rikke as per usual and she now sends you to Fort Kastav. Fast travel to the nearest location and make your way to the marker.