Cut out three of the shapes.
Now for the fun part. Find the fabric you want to make your Kai No Kuchi purse out of, fussy cut, place, whatever and make sure your fabric is doubled over. You will need a back and front for each of your three shapes and everything will need a 1/4" seam.
There seems to be a couple of different way to do this next bit but as I'd marked the seam allowance already, it made it easy to sew along one side and about 1cm (1/2") on both ends of the other side. I clipped the sewn edges and turned the shape inside out and then put my plastic shape inside it.
Once all three shapes are covered it's time to sew the shapes together, the final shape of the purse is a triangle shape.
Here is two pieces sewn together and here is me getting the last piece ready. The plastic piece is inside and I was sewing the fabric together to get it ready for sewing to the other pieces.
All done, all three pieces are sewed together.
Viola. Finished. The rigidity of the plastic keeps the little purse rigid and keeps the purse closed. To open it, all you need to do is gently push the two pointy ends and the purse opens. I also sewed up 1cm (1/2") on each end to make the opening a bit more secure. Its a great project and it is really, really, really easy!