You can add a Fancy Foil cover to a Classic Book you're already making, or start a new book with one. For sizes, pricing, and title details, see What are Fancy Foil covers?
How do I add a Fancy Foil cover to an existing Classic Book?
Tap on the cover of your Classic Book
Tap "Change Cover"
Tap "Add a Designer cover"
Choose a cover that has "Foil" in the title, then tap "Select cover"
The cover will apply to your Chatbook, then tap "Done"
How do I start a new Classic Book with a Fancy Foil cover?
From the Create screen, tap "Foil Covers"
Tap on the cover you'd like for your Chatbook
Tap "New Book"
Select the size of the book and add your photos
To swap to a different Fancy Foil cover, follow the steps above