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: Party Prep. You can begin this quest in the Tundra Express location after receiving two Damsel rockets from Tina during the main mission named A Train to Catch.
2) Madame Von Bartlesby/Sir Reginald
3) Bandit camp/Crumpets
4) Princess Fluffybutt
Tina  will ask you for help in preparing a party. Firstly you will need to visit Sir Reginald. Head to the hill , where you will have to defeat Madame Von Bartlesby from the above screenshot.
After the fight, approach the nest and take Sir Reginald. The second step involves collecting three parts from Buzzard wrecks. Machines will fly out of hangars in the Bandit camp , see above screenshot for reference.
In the camp you can also find Crumpets, collecting which can give you additional experience points.
Now you need to get Princess Fluffybutt. She can be found on a chair nearby a single building . Having all the items, return to Tina  and place the "guests" on their proper places. Finish the mission by speaking to Tina.