You want to start in the middle of the book. For a 300 page book, start at page 100. Simply fold the upper right corner of the book page about 1/4" down to meet the center.
For the 300 page book you will have 100 unfolded pages in the front, 100 folded pages in the middle and 100 unfolded pages at the end. So continue to repeat these same 3 folds for the remaining 99 pages.
For the hardcover book I decorated the inside covers with a printable Christmas image with a red cardinal. You can download the image for free on my blog with the link below.