From your Classic Book:
Tap and hold your finger down on a photo
Select 2-6 images and tap "Collage"
Note: collage options are available for 2, 3, 4, and 6 images.
Tap "Done Rearranging" to view your collage in your Classic book
To edit the collage:
Tap on the collage page
Tap "Edit collage"
From here you can convert the photos back to individual pages OR
Tap on a photo to crop, or change to a different image
You can also hold down on an image within the collage and move it to a different location in the collage layout
Once your images are cropped and in the location you'd like tap "Save"
iOS:
How to make collage pages for Classic Books