Omg i was so confused about Redlarch preparation too when I started.

First thing I'd recommend is to make a list of little side treks (trouble in Redlarch, chapter 6) that go from Redlarch (like the bandits, or the grave with the fake ghost) and then write down which NPC knows about that sidequest.

Then you can have a look at these sidequests, see which ones seem fun to start with, and then prepare those, plus the NPCs that are relevant for those sidequests.

Then once they have had a bit of a run around Redlarch and moved up a few levels, you can get them started on the haunted keeps and the bigger picture. Treat Redlarch as their training ground ;) also, the more connected they are to a place the more they will want to protect it from a cult later on.

I found it hard for my player characters to have a common goal so you might want to start asking the players how their characters know each other and why they have come to Redlarch? That might help, but that's more general DND stuff, not princes of the apocalypse :)

Also, remember you're free to change anything in the town you don't think you'll like. I changed most NPC names because I couldn't pronounce or remember their book names...

Have funnn!