The First Step

Go to the station and talk to marked settler to update your quest. Next, you have to head to the Corvega Assembly Plant. Note: If you have to Corvega Storage Key, you'll be able to get additional loot from that place. Otherwise, return to Concord and check for every last raider body you killed earlier during the second quest. (When Freedom Calls)

Corvega Assembly Plant is really huge so it will be hard to get a proper walkthrough for it. Explore it as you can. The boss of the raiders is located on the upper floors so find your way there. Be careful of the atomic cas in the assembly since they'll still cause a small nuclear explosion when detonated. The storage area where you can use the key is located in the main floor in the middle, which you can easily get into if you entered from the Lexington side of the factory.

There's also a Repair Bobblehead that you can get from the rooftop. Just look for any "Exit" doors in the upper levels leading back outside. From there, you have to make your way to the large blue sphere to the south. The bobblehead is found at the end of the topmost walkway.

You don't need to kill all raiders in the building. You just have to reach the boss (Jared) and kill him. If the quest will still require you to kill more raiders, just follow the markers and you will be Lead to them. Getting the Jared's body and the control room may be a bit tricky.

To reach it, you have to jump over the gap from the catwalk in the corner of the assembly line, next to the control room. Equip something lighter, like a pistol or just simply hide your weapon. Make sure to hold the jump button since it seems to make your jump stronger. You can refer to the video below:

After reaching the control room, you can now loot a steamer trunk containing ammo and weapons and pick up the Grognak the Barbarian comic on the table.

Once done, leave the factory and fast-travel to Oberland Station. Report your success and Oberland Station will be another new settlement for you to manage. Now return to Sanctuary and talk to Garvey to receive your reward and start the next quest. Garvey will also become available as a companion.