Ask for paper bags instead of plastic when you food shop. Lay the paper bag flat on a clean table. Using both hands, smooth out any noticeable creases and/or wrinkles on each bag. Get your books, and place them on the table near the brown paper.
Take out your scissors and carefully cut off the thick bottom of the shopping bag. Cut through the thick bottom of the bag and discard. Do the same thing for each bag that needs to become a book cover. Open and spread them out on top of the table.
Take one of the books and place it in the center of the brown paper. Lift the paper ends up against the side of the book bottom, and use your thumbnails to create a crease line along the paper (see illustrations in Resources below). Do the same thing for the bottom of the book. Remove the book.
Use the fingers of both your hands and firmly flatten the crease line you just made. Use the side of your thumbnail to sharpen the crease line (on the top and bottom lines}, and fold them down towards the paper. This will now be the width of the book. Center the book on the top of the paper and open the covers. If it is not centered, then move it around until it is.
Lift the end of one side of the book cover, and slide it into the slot created when you folded it. Do the same thing for the other side of the book. Each end of the book cover should slide into the slits smoothly. Adjust if necessary. Keep smoothing the paper throughout the process while opening and closing the cover making sure the book opens flat.