On Youtube there is a guy who has a good tutorial on how to do it.
Title: How to Print Sections or Signatures from a PDF File for Bookbinding // Adventures in Bookbinding
Channel: DAS bookbinding
Summary: Basically you need to make each signature a new document, and then print each of these documents as a booklet.
Probably don't even need affinity for this one. Honestly, it's annoying that publisher doesn't have this, seems like something that is relatively easy to implement and makes sense for a publishing software to be able to print the signatures out and make a book.
Cheers!
