Instructions:

1)  Start with an ordinary lunch bag....
2)  Set the bag standing upright and cut down one corner of the bag     down to the bottom fold....
3)  Now cut along the bottom of the bag and remove the bottom.
4)  Lay out the bag, long sides on top and bottom.
5)  Fold the top down to meet the bottom and using your bone folder crease well.
6)  Open up the bag and now fold the short sides together and crease.
7)  Open up the bag and now fold the short sides on both sides to meet the center crease.
8)  Now push the short sides to the center forcing the center to create and mountain fold.  Looking at it from the end, it will look like a "W".
9)  Now fold the side flaps down with the center still elevated.  With your scissors, cut a slit on the middle fold line of the center of your "W" down to where the side flaps meet it. 
10)  Open up you lunch bag and holding both short sides, fold the top long side to the back down to the bottom long side.  The slit you cut should now be on the top.  Holding onto both short sides, push them together so that the front center crease comes forward and the back center crease goes back.  It should look like an"X".  You have just formed your 4 pages of your book.  Besure to crease you pages with a bone folder to shape them.
10)  To strengthen your book you make want to put some adhesive inbetween the 2 sheets of each page.
11)  NOW, the fun begins.  You can start decorating!
I used acrylic paint on my pages.  It adds a little strength and gives a great decorative backing for your stamping and other techniques that you want to use on your pages.