1. Take A4 paper, cut a square with a size of 21x21 centimeters.
2. Then bend the sheet diagonally.
3. Bend the paper again, resulting in a triangle.
4. Then open the sheet completely and fold the paper in relation to the intersection of the diagonals.
5. The result is a two-layer square.
6. Bend its sides in half, and then along the same lines raise the sides of the future box.
7. For strength, you can connect the sides of the square with glue or stapler brackets.
This will be the bottom of the box, in the same way you can make the top.