On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled You Are Cordially Invited: RSVP. You can begin this quest in the Tundra Express location after the completion of the side mission named You Are Cordially Invited: Party Prep.
2) Bandit camp/Flesh-Stick
After talking with Tina , head to the Bandit camp . Clear the area of enemies for Flesh-Stick to appear. You cannot kill him or the mission will end with failure. When Flesh-Stick heads out of his hideout, shoot beside him a couple times so that he start to chase you. Lure him towards Tina's hideout . If for any reason he stops, shoot around him a couple times again. If you have a weak weapon, you can just shoot him directly.
When you reach the cave, Flesh-Stick will be caught into a trap. Return to Tina to finish this part of the mission and choose one of the prices.