A square sheet of paper.
Fold it once.
Fold it twice.
Turn your paper around.
Fold the corners to the center.
All of them.
Fold the sides to the center.
And now fold the other two sides to the center.
Unfold one of the upper and one of the lower triangles.
Fold the sides to the center (again).
Breathe. It should look like this now. (Almost done.)
Half unfold the sides.
Now the hardest part: lift the triangle up -
- so that the inner folds meet in the middle.
Wrap the triangle over the side.
Do the same with the other end of your box.