Step one is to seal the bulk of the pages and back cover of the book into a solid chunk, leaving the front cover and a few pages loose.
Gather your supplies and tools, then select the starting point for your hollowing -- typically, you'll want to leave a few uncut pages at the beginning.
Insert your Mylar sheet or cling wrap at the page where you'll be cutting in order to mask the front cover and loose pages from the glue you'll use.
Using the brush, apply a liberal amount of glue around all of the edges of the pages. There's no need to put glue in between each page -- the glue will soak in a bit and bind the pages, as well as the back cover.
Once you've glued the page edges on all three sides, it's time to press the book closed firmly and allow it time to dry.
Ideally, you'll use a set of spring clamps or c-clamps to hold the book closed during drying, about 30 minutes will do.
If you don't have any clamps, stack some very heavy objects on top of the book while it is sitting on a hard, level surface. You want the pages compressed together tightly. If you can't find anything suitably heavy, it's fine to put the book between to boards and sit on it. Seriously.
Once the glue has dried for at least 30 minutes, carefully open the front cover and loose pages, then remove the Mylar sheet or cling wrap. If there is any excess glue around the edges, trim it away with a hobby knife.
Using a straight edge/square and a pencil, mark the rectangle you'll cut for your hollow section.
Use the straight edge as a guide, pushing down firmly, and then cut a few pages deep with the utility knife. You don't need to try to cut too deeply, this process works best if you cut moderate sections at a time.
Make a cut along each edge of your rectangle, then lift up and remove the cut section of paper.
Avoid pulling the paper out if it is stuck at a corner, and instead cut with the knife -- the small leftover corner bits can start to build up!
After ten minutes or so, you should be at a pretty nice depth into the hollow. Avoid cutting down into the back cover! If you know of a specific object you'll be hiding in your book safe, make sure to test its depth as you're working so that the hollow doesn't come up shallow.
Once you've gotten the hollow where you want it, it's time to seal the interior. Again, using your small brush, apply the Mod Podge or white glue to the page edges that form the inside walls.
Once you've sealed up the walls of the hollow, insert the Mylar sheet or cling wrap between the hollow page and the loose, uncut pages. This will protect the loose page from getting stuck to the glue while it's drying.
Yes, it's that time again -- Clamp Time! Close the book, then place it between two boards or books and clamp it all down tightly. C-clamps work really well for this, because we can generate a lot of clamping pressure. The boards help distribute the pressure evenly, in order to flatten the inside edges of the hollow, which tend to curl up during cutting and gluing.
Let the glue dry for at least a half an hour, then remove the clamps and boards. Carefully peel away the protective mylar sheet or cling wrap.
Your hollow book is ready for use! You can now hide your secret items in it and place it upon a shelf, or, go to the next page to learn to build a Circuit Playground Express alarm!