I have seen one made like this, if it's the one on creat and craft a uk channel.
If it is that one you fold the card in half as normal, mark the centre of the front edge.
Score from the card middle to the centre front.
Now measure 2 cm from the centre fold of the card, mark this. Mark 1cm each side of the centre on the front edge. Score these lines.
It makes an wedge which you just press in gently to make the shape.