Meet the men near Fort Neugrad
Make your way to the marked location. You can travel to Helgen or any nearly location. From there, just make your way to the marker where you will find the gathered Imperial Legion force waiting for you to arrive.
Take over the fort by defeating the enemy
Like with any fort conquering, you will need to make your way inside the Fort. As always, start by clearing out the archers on top of the walls that are helping to protect the entrance.
It is best to peruse this using a methodical approach. Take your time clearing out each section of the Fort. Start with the courtyard if you are melee focused and have weapons and potions aplenty. Otherwise, let the Legion engage the courtyard and you should take to the walls. Going up onto the walls will let you eliminate the archers that are firing down into the melee on you. Take the approach that works the best for the build and play style that you have for your character.