Step 1: Find or make proper scale pages. I did mine in photoshop. First thing I did was pick the scale of the doll I wanted to make them for, then downsized the human versions to that percentage. So, these pages are 1/4 scale (approximately, as I made a few adjustments so they'd look right).
Step 2: When you have the pages printed you want to use, or if you just want to use blank paper, cut them to scale. Make a neat stack.
Step 3: Apply craft glue, like Aileen's tacky glue (something with a little bit of give that won't snap or tear when you open and bend the spine of the book) to the edge. Let it dry thoroughly. When dry, even the other edges up with scissors or a craft knife.
Step 4: Add your lovely printed cover.
Done! You can also use a method similar to this one here. I'm working on making some tiny dice to go with it, or at the very least trying to find some good in-scale dice.
Here is my Hugin with character sheets printed using this method:
And a close up: