Fast travel to the nearest location and make your way to the rest of the Imperial Soldiers. Once you reach them, it's practically the same as the other quests: take down the Fort while you kill all the Stormcloaks. There are obviously more enemies in this fort, so I really recommend going to the upper area and staying here.
If you can summon, do so. Familiars, especially Atronachs, are wonderful allies here. Most of the Stormcloaks appear from the upper areas, so this is where you'll be. Make sure to have your healing spells on your favorite list, or Restore HP potions. Not too hard, but don't do what I tend to do and forget about the HP bar! Once the fort is taken; return to Rikke and tell her about your success.