On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled The Overlooked: Shields Up. You can begin this quest in The Highlands after the completion of the side mission named The Overlooked: Medicine Man.
2) The grinder
After you accept the mission, you will have to drop five shields into the grinder  from the above screenshot. If you don't have enough of them, you can buy some from the nearby Vending machine. Destroy the shields, pick up the Shield Core and bring it beside Karima's house. Speak to her through the door to end the mission.