On this page of the game guide to Borderlands 2 you will find the walkthrough description of the main mission titled The Once and Future Slab. After meeting Roland in the Sanctuary, the hero must embark on a journey to the Thousand Cuts location.
2) Bandit camp
3) The Slab King hideout
Take Roland's Note from Roland . Use Fast Travel downtown and teleport to The Highlands Hyperion Bridge. Head to the western part of the map, where you will find a passage leading to Thousand Cuts. Enter the first Bandit camp , eliminate all the enemies and head north.
Fight your way through the camps and eventually you will reach The Slab King hideout . Before you use the stairs on the left, head to the right side of the building to find a ladder from the above screenshot . Below you will find a chest.
Now return to the entrance to The Slab King hideout  and go up the stairs on the left. there you should find a descent visible in the screenshot.
Below you will have to fight a couple Bandits. In the end, take care of Slab King's underlings (the passage up is in the left side of the room). With just Slab King left in the room, give him Roland's Note.
Head out through the door which Brick will open. Start following him and destroy all the Robots that you come across. If any is left, Brick might "get stuck". While you're taking care of the Robots, look out not to stand on the marked from the above screenshot.
Your goal will be destroying three transmitters. Wait for Brick to destroy the target's shield and destroy it afterwards. With all three transmitters and Robots destroyed, use Fast Travel and return to Sanctuary. Speak to Roland  to end the mission.