Start with pages that you've either cut openings in (descending in size from large to small) or used a set of nesting dies on. For mine, I used a label set of dies from Spellbinders.
Using a scoring board, make two identical accordion-folded sides. It doesn't matter how close together the folds are or how many folds you have. It's entirely up to you and how many levels or layers you want your tunnel book to have.
Glue your pages to your accordion sides. Here are pictures of one side connected to the pages:
Now you're ready to make your background scene (what you'll see through the tunnel holes). I made a fabric collage of a bird.
Attach the bird scene to the back page and glue on your second accordion side. Unlike a real book, tunnel books typically don't open. Both sides of the pages are glued to the accordion sides so it's more like a tunnel box. Here's what it looks like from a few different angels once I glued it all together. I also added a Prima flower and a strip of lace:
After it was glued together, it was easy to look and see where else I wanted to add embellishments. I added some other flowers, yarn and some fabric strips to look like curtains on the sides. Here is my finished tunnel book.