On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled Stalker of Stalkers. You can begin this quest in The Highlands during the completion of the main mission named Bright Lights, Flying City (the successful defence of the Beacon in The Highlands is a requirement for this quest).
1) Bounty board
2) Stalkers' terrain
3) Badass Pyre Thresher
You can receive this mission from the Bounty Board  in the northern part of Highlands. You will have to search Stalker food remains shown in the above screenshot , found in the area inhabited by those animals. After searching the proper remains, you should find a ECHO Recorder. Be very thorough and search every pile, including the ones hanging from rocks. Apart from three places in the northern part of the map, there's also one in the south. However if you're lucky enough, you might not have to go there. Additional experience points can be received for killing fifteen Stalkers.
Be sure not to approach the above house when searching the north-east nest . If you get too close, you might be attacked by a Badass Pyre Thresher.
After finding five recordings, approach the mailbox in the camp  to end the mission.