Step 11: Fold the inner triangle over to the right side
Step 12: Fold the top of the plane down to meet the bottom and press down the crease
Step 13: Fold the outer top corners inwards and diagonally down to meet the centre ‘cross’ of the previously made crease marks (indicated in red)
Step 14: Your plane should now look like this
Step 15: Open up the paper
Step 16: Fold the inner section upwards, creating the crease line indicated in red
Step 17: Your plane should now look like this
Step 18: Fold the outer ‘wings’ in half, folding them in to meet the diagonal crease lines
Step 19: Fold the outer ‘wings’ over towards the centre once more
Step 20: Fold the top of the plane downwards, creating the crease line shown in red