On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled Uncle Teddy. You can begin completing this quest during your first visit to the Arid Nexus Badlands location. This happens right after you complete the storyline mission named Data Mining.
Arid Nexus Badlands
1) Bounty Board/Una Baha mailbox
2) Tedd's house
3) Hyperion mailbox
Accept the mission from the Bounty Board  and head to where Tedd lives . Head inside and pull the fan chain shown in the screenshot. That way you will open a descent to the secret room below the building.
Below, start searching all the lockers and boxes. In chosen lockers, you will find six ECHO Recorders. After finding all of them, approach the Vending Machine. Pull the lever on the left (screenshot) and take TK's Blueprint from the Machine. Now you will have two options: you can either throw the weapon plans into Hyperion's  (in return for a shotgun) or to Una Baha's box  (you'll get a pistol).