Step 1:
Both papers used for this model are the same size.
Fold in half.

Step 6:
Prefold in half over this line.

Step 5:
Valley-fold over this line.

Step 2:
Prefold in half over this line.

Step 4:
Fold the edge to the prefolded line.

Step 3:
Valley-fold over the upper line and prefold over the line below (fold the edges to the centerline).

Step 10:
Turn the model.

Step 8:
Prefold over these lines, they are exactly in the center of the previous prefolded lines.

Step 9:
Fold over these lines. The bottoms of the diagonal lines are exactly in the center of the prefolded lines.

Step 7:
Prefold over these two lines (fold the edges to the centerline).
Then unfold the fold of step 5

Step 16:
Prefold in half over this line.

Step 15:
Repeat steps 1 to 4 on a second paper. Step 4 is a prefold line now instead of a normal fold, this will make the upper part of the box a little longer.
Valley-fold over this line.

Step 12:
Repeat step 11 three more times, see arrows.

Step 11:
Press the part on the right (see arrow) over the part on the left.

Step 14:
The bottom-part of the box is done.....

Step 13:
Tape or glue both edges together, see arrows.

Step 21:
Press the part on the right (see arrow) over the part on the left.

Step 19:
Fold over these lines. The tops of the diagonal lines are exactly in the center of the prefolded lines.

Step 20:
Turn the model.

Step 18:
Prefold over these lines, they are exactly in the center of the prefolded lines.

Step 17:
Prefold over these two lines, fold the edges to the centerline.
Then unfold the fold of step 15

Step 22:
Repeat step 21 three more times, see pink arrows.

Step 24:
And the upper part of the box is ready too.

Step 23:
Tape or glue both edges together, see arrows.