Prepare the paper and surface
Choose a square sheet around fifteen centimetres on each side. Very thick paper can be difficult to fold; ordinary paper is sufficient for experimenting. Work on a clean table and crease with a fingernail or the side of a finger. If the paper has two colours, decide which should show outside. The first aim is to understand the sequence without demanding a perfect result on your first attempt.
Make a triangle with the opening at the top
Place the square in front of you with one corner pointing down, like a diamond. Bring the bottom corner to the top and crease to form a triangle. The long base is at the bottom and the point with two layers is at the top. Distinguishing these layers matters for the next step. If the corners do not align well, unfold and correct them now: a small initial adjustment makes the final folds easier.
Make the side creases
Bring the right corner of the base to the top point and crease. Repeat with the left corner. You now have something resembling a small diamond. Undo only these last two folds to return to the triangle. The creases remain as guides. Now fold down only the front layer of the top point, bringing its tip approximately to the centre of the base. The back layer stays upright.
Close the little pocket
Bring the right corner of the base up again along the crease already made. Tuck its end behind the front flap you folded down and into the small pocket. Repeat on the left. Smooth carefully. The resulting piece has an opening that can fit over a page corner. If a tip will not go in, do not force it: check you lifted the correct flap and kept the back layer free.
Decorate without damaging the book
Try the bookmark on a loose sheet first. It should slide without catching or forcing the paper to bend. Draw small patterns outside if desired, but avoid wet ink, bulky decorations or staining materials. If the fit is tight, remake it with a larger square. The exercise combines usefulness and observation: after each fold, check where the opening is. Attention to orientation is a useful foundation for trying more complex models.
