Instructions
Travel to the small community of Girdershade. If you have activated the un-marked side quest "Grady's Package," your PIP Boy will display a map marker for Girdershade. Otherwise, you will have to discover it by exploring the Wasteland. The community is located in the southwest corner of the world map, approximately two grid boxes from the western edge and four grid boxes from the southern edge.
Enter the northern-most of the two houses in Girdershade. Inside, you will encounter the non-player character Sierra Petrovita, an extremely enthusiastic Nuka Cola addict. Initiate dialog with Sierra.
Accept Sierra's offer to give you a tour of her Nuka Cola "museum." Do not break off dialog during the course of the tour. At the conclusion of the tour, she awards you an Ice-Cold Nuka Cola, then asks for a favor.
Follow the dialog options that allow Sierra to ask her favor. She requests that you bring her 30 Nuka Cola Quantums -- enough to fill her Nuka Cola machine. In return, she promises to pay you 40 caps for each bottle, plus share "a big secret" if you bring her 30 bottles.
Accept Sierra's offer to initiate the quest "The Nuka Cola Challenge."