Take a square sheet of paper
Fold it in half diagonally
Unfold it
Fold the left and right edges toward the center crease
Fold the shape in half along the center crease
Fold the top corners inward to form a pointed tip
Repeat the same folding steps with additional paper sheets if you want a thicker blade
Make a second paper strip for the handle
Fold the strip lengthwise several times until it is firm
Insert the blade section into the handle section
Secure the joints by tucking the paper edges tightly
Flatten and adjust the shape so it looks like a kunai