Measure the length and width of the front or back cover of a specific book. Measure the spine of the book as well.
Draw a rectangle that represents the dimensions of the book's spine on a sheet of sturdy paper. Draw two rectangles or squares that represent the dimensions of the front and back of your book to the left and right of the spine rectangle. Draw two more rectangles or squares on far left and right to create flaps. The flap should be the same length as your book cover and about a quarter to a third of the width of the cover.
Cut out the drawing that you created.
Center the spine of the book on the outline that represents the spine on your book sleeve.
Lay the back cover of the book flat on a surface and flip all the pages of the book toward the front cover. Fold the flap on the left side over the back cover.
Lay the front cover down flat and flip the pages toward the back. Fold the flap on the right side over the front cover.
Decorate your cover if you wish with markers, pencils, pens, crayons or stickers. Remove the cover before decorating and reattach it to the book once you've finished and any ink you use is dry.