Get an old book, open the first two pages to the left side (we do not want to play with them right now) and draw a rectangle with your pencil to the right page.
With your craft knife and a metal ruler start cutting the rectangle along the lines until you get the desired depth, almost the 2/3rds of your book (don't cut all the way through!)
So you end up having something like this...
Put a grease proof paper (like the one you are using in your kitchen) between the two pages you left uncut and the main book body and another grease proof paper at the end of the book and the book cover. The grease proof paper doesn't allow the glue to go into places you do not want (like the book covers!). With a flat brush, apply a generous amount of glue to the main book body all three sides). Put a weight on top of your book (a stack of magazines or books would do) and leave it to dry completely. Depending on the thickness of your book you might need to apply a second coating.
After it is completely dry open the book and turn the first uncut page you left to the left side and glue it down to your book cover (in my photo i have accidentaly cut that page, please ignore the hole in mine!).
Turn the second uncut page you left to the right and glue it over the hole you have made. The with a craft knife cut the same rectangle. This will give a crisp cut of your shape and will cover any uneven cuts of your whole.
Paint with any medium you want both sides and the hole! Here I have used acrylic paints and glimmer mists.
Cut patterned paper and glue down as desired and start decorating your inner pages. It is really interesting to give dimension to your hole by glueing down in different levels your embellishments. It is easy to lift slightly some of the inner pages in the hole and create a 3D feeling out of it like I did here with the clouds and the grass.