How to fold the cardboard.
You will need a ruler and a bone. First, mark the future crease from the inside. Then, keeping the ruler in the same place, start bending the cardboard towards you with a bone. The thinker the cardboard, the longer this step may take. And finally, use the bone to even out the crease from both sides.
How to prepare the bookbinding string.
First you string need to be waxed, so that it does not reap the paper and is more protected from the moisture. Then pool your string the the needle ear, thread the shorter side of the string through the needle and pull on the longer side to fixate it. Pull on each side until the string sits firmly in place. Now you can start bookbinding!