Fold the corners of the pastry towards the middle of the pastry and brush them with the egg yolk mixture as well (see the picture above). (whereismyspoon.co)
Then take the bottom corner and fold it up and fold the corners over. (rezelkealoha.com)
Brush the corners of each square with a bit of water, then fold each corner into the centre and press down gently. (seitanismymotor.com)