Galmar has sent some scouts out ahead of us, and we're supposed to meet them. The plan is to sneak into the fort's prison and let the prisoners out so we can take over the place from the inside. Brilliant! Go find Ralof (the scout) and talk to him. He'll state that they found a cave that leads to the prison under the lake nearby the fort that you can use to sneak in. Even if you're not much for sneaking, he still says that's the way to do it.
Note: Cast Muffle and keep to the right side of the fort, sneaked down by the pier, and into the water. There may be someone on the pier, but he shouldn't notice you. We did that at a Sneak skill ofat 60, and my Illusion (Muffle) at 37.
If you CAN manage it, the cave is underwater and leads directly to the prison. Here you can shank the prison guard and use his key to let four prisoners loose. They will then go to a nearby chest and gear up, and help you attack the fort. Follow them outside as they and Ralof's men clean up.
From there, head inside of the fort proper (only Ralof followed me in) and start to kill the Imperials inside. Be sure to loot the place, as there are a lot of potions laying about. There's also a LIGHT ARMOR SKILLBOOK lying on a table through the middle door of the main room called "Jornibret's Last Dance". Once you've killed all the guards and the fort commander (who isn't too hard) your objective will change to "Talk to Ralof". Once you talk to him, he will tell you to go report to Ulfric and this mission will be complete.