So, tomorrow I am supposed to GM for a group of girls that are basically Slaanesh cultists in real life. All of them experienced RPG players.

The fact is, they've asked me to GM it since they saw my book, and although I have extensive experience with OW and RT, I have never GMed or played in a BC campaign or One-shot session.

This could potentially become a campaign, but I am quite at a loss on how to conduct it. I guess I will just adapt that starter mission to be more slaneshi and go with it.

I need your thoughts and ideas, including on the PCs I will be creating for them in advance.

The adventure included in the core rulebook can be given a flavour more "desirable" to Slaanesh followers. There is already a story hook you can use regarding an ancient book. You can multiply the items of interest by 4 or mix it up to include things you'd get a Rogue Trader to procure I gather that from your OW and RT experience you're rather familiar with what Slaanesh followers gravitate to. Just use that knowledge as a start and be extra liberal about how they go about achieving things. You'll get your barrings eventually and make a more solid structure in later sessions (if you need one).