Step 1:
Prefold in half two times

Step 6:
Open the model again (=unfold the folds of step 3 to 5).

Step 5:
Fold in half one more time.

Step 2:
Fold in half over the prefolded line.

Step 4:
Fold in half

Step 3:
Fold in half

Step 10:
Fold the part on the left to the right, use the crossing point of the edges as guidance, see arrows.

Step 8:
Fold the model in half.

Step 9:
Fold the upper flap upwards.

Step 7:
Fold over these two prefolded lines.

Step 16:
Fold over the upper line first, then the line below.

Step 15:
Fold the corner backwards, see short purple line.

Step 12:
Fold the corner on the left to the corner on the right

Step 11:
Valley-fold along the edge

Step 14:
Turn / rotate the model, see next picture.

Step 13:
Fold the part on the right of the purple line to the left (=fold in half)