Take your card and score a line halfway horizontally and draw a line halfway vertically;
Cut a line from the top of the card to the horizontal score line 2-3 mm each side of your vertically drawn line and then cut across the score line between the two cuts to remove a "notch";
Score the vertical line along the drawn line from the horizontal scoreline.
Fold the two top squares down along the horizontal scoreline to make a 12" wide 6" high rectangle and then fold the rectangle in half along the vertical scoreline to make your book.
That's it easy right?
The notch is needed so that the book still folds easily once embellished with papers and photos and you can not use anything too dimensional on the inner pages. If you glue a piece of ribbon along the back page before you stick the papers etc on then this can be used to tie your book closed.
For a finished example see here