First you will make a Duct Tape sheet longer that your book when it is open plus extra for flaps.
For this, cut multiple sheets of Duct tape longer than the length of your book while open plus 4-8 inches (depending on how thick you would like your flaps that will hold your book in its cover). Place your first strip sticky side up. Next place another sheet sticky side down vertically across your first piece and halfway up so that there is equal sticky overlapping on top and bottom. Flip the strip over and with your next piece place it either below or above the second strip of tape so that the sticky area is completely covered. This will leave another half piece of sticky overlap.
Repeat this process over and over until you your Duct tape sheet is about a strip and a half wider than your book is top to bottom. Fold over your sticky pieces and give the top and bottom of your cover a clean look.
Cut the ugly ends of your duct tape book cover so that they are clean as well. Now, place your open book in the middle of the sheet so that the flaps are equal on both sides. Now fold the flaps tight over the front and back covers of your book and tape the top and bottom parts of only your flaps (this way the hard part of your book can easily slide in and out of the Duct tape flaps.
That is it. You have a Duct tape book cover!