Step 1:
Prefold in half over both lines.

Step 6:
Valley-fold over these 3 lines (=rabbit-ear fold). Use the prefolded squares pattern as guidance.

Step 5:
Repeat step 3 and 4 in this direction.

Step 2:
Fold the corners to the center.

Step 4:
Fold over these two lines and unfold the folds of step 3 and 4 afterwards.

Step 3:
Fold over these two lines.

Step 10:
Fold the center into a square shape.

Step 8:
Fold over these three lines, use the prefolded squares pattern as guidance.

Step 9:
Repeat step 8 on these three places.

Step 7:
Repeat step 6 on the other three flaps.

Step 12:
Fold the flap in half two times. Then fold it downwards over the lower line.
Repeat on the other three flaps.

Step 11:
Fold the edge inwards on these four places, use tape or glue.