Simply complete 5 missions started in the Rust Commons. You can easily earn this just by playing out the main storyline. However, below is a complete list of all Rust Commons' missions that you can choose from:
Claptrap Rescue: New Haven
Claptrap Rescue: Tetanus Warren
Corrosive Crystal Harvest
King Tossing
Is T.K. O.K.?
Like A Moth To Flame
Power To The People
Up To Our Ears
Scooter's Used Car Parts
Hidden Journal: Rust Commons West
Scavenger: Submachine Gun
Firepower: All Sales Are Final
Firepower: Market Correction
Firepower: Plight Of The Middleman
Jack's Other Eye
Seek Out Tannis
Meet "Crazy" Earl
Claptrap Rescue: Scrapyard
Get Off My Lawn!
Today's Lesson: High Explosives
Hair Of The Dog
Claptrap Rescue: Krom's Canyon
Earl Needs Food Badly
Missing Persons
Two Wrongs Make A Right
Middle Of Nowhere No More: Investigate
Middle Of Nowhere No More: Fuses? Really?
Middle Of Nowhere No More: Small Favor
Middle Of Nowhere No More: Scoot On Back
Scavenger: Shotgun
Scavenger : Machine Gun*
Altar Ego: Burning Heresy
Circle Of Slaughter: Meat And Greet
Circle Of Slaughter: Round 1
Circle Of Slaughter: Round 2
Circle Of Slaughter: Final Round
Hidden Journal: Rust Commons East
The Next Piece
Altar Ego: The New Religion
Altar Ego: Godless Monsters
A Bug Problem
Relight The Beacons
Green Thumb
Smoke Signals: Investigate Old Haven
Smoke Signals: Shut Them Down
Claptrap Rescue: Old Haven
Bandit Treasure: Three Corpses. Three Keys.
Bandit Treasure: X Marks The Spot
Jaynistown: Spread The Word
Jaynistown: Getting What's Coming To You
I've Got A Sinking Feeling
Wanted: Fresh Fish
Jaynistown: Unintended Consequences
Jaynistown: Cleaning Up Your Mess
Bait And Switch
Claptrap Rescue: Trash Coast
Earl's Best Friend
House Hunting
Another Piece Of The Puzzle
Not Without My Claptrap
Happy Hunting :)