That is a dramatic LO!
Well I'd start by measuring corner to corner, so a 12x12 page would be 17", and halve that: 8.5". I'd then cut strips into points, making sure that the shortest edge was longer than 8.5". I'd arrange them on the page so that the outer edge overlapped the edge of the card-stock and stick them down. If the edges overlapped I don't think it would matter. I would turn the page over and use a knife to trim along the edge of the card-stock, then stick my photo in the middle to cover the centre part.
Is that what you mean?