Step 1:
Prefold in half, then fold both parts in half three times. Unfold the model afterwards.

Step 6:
Fold over the white line ( half a prefolded square from the edge ) and prefold over the other lines.

Step 5:
This folding line is 1,5 prefolded squares from the edge.

Step 2:
Repeat step 1 in this direction.

Step 4:
Fold over this diagonal line ( 4x4 prefolded squares ).

Step 3:
Fold over the diagonal line ( 6x6 prefolded squares ). Then fold the model in half over the prefolded centerline.

Step 10:
Make sharp(er) folding lines and glue / tape the right edge over the left edge, see arrow.

Step 8:
Prefold over the white lines by folding the yellow lines next to them towards each other.

Step 9:
Glue / tape these edges together.

Step 7:
Prefold over these lines, use the prefolded squares pattern as guidance.

Step 12:
Fold the back of the board flat, make the same folds on the other side, see arrow.

Step 11:
This is how the model should look now.