This quest is started at the Sunset Sarsaparilla Headquarters, which is located on the map Northwest of the vegas gate, just south of the Monte Carlo Suites, and just west of the NCR Sharecropper farms. When you get there enter the factory floor through the door with the giant soda bottle over it. On your local map you can see that this door stands alone while there are two other doors right behind each other. Head inside and walk a little bit forward and then Festus will be on the left. He's a puppet/vending machine. Ask him about star caps to get the quest. Now the objective is to find 50 starred Sunset Sarsaparilla bottle caps.
Here's a list of all the cap locations -
http://fallout.wikia.com/wiki/The_Legend_of_the_StarOnce you have 50 starred caps in your inventory head back to Festus and talk to him. It will then say you failed the quest, but give you a new quest called "A Valuable Lesson" follow the marker and get your reward for finding the caps, which will complete the quest and give you your achievement.