Hotglue the two bladed edges together at the front. (Being careful to glue small sections of it at a time, holding them closed as they cool.) Trim off any excess plastic which may overlap the blade edge.
Next, take a thin strip of plastic and hotglue it around the outer edges of the blade, making sure you cover every surface (aside from the area around the ring. Don't glue any plastic around the ring part of the blade; the craft foam is really too thin for the lack of plastic on the edge to be noticable once the shuriken has been painted.)
With a craft knife, cut away any excess plastic which may overlap the blade edge. Finish the edges so that they're nice and smooth.
You now have one finished blade. Before painting the blade, sand it down with hobby/model sandpaper (not regular sandpaper; it's too rough) so the surface is nice and even and free of strands of glue and/or dirt. Repeat the process until you have 4 finished blades.
Now you have to create the center rivet that all the blades will swing around. First start by taking a long strip of plastic about 1-1/2" wide and gluing it into a coil. Make sure the coil fits into the blade rings; (make the coil just big enough so that the rings will move around the coil without slipping.)
The rivet will consist of the inner coil with two flat rings at the edges (so it will look just like a spool). Cut one circular ring out of plastic and one out of 3mm craft foam and glue them together. The inner edge of the ring should just fit onto the edge of the inner coil. (What you're doing, basically, is making one half of the spool.)
Once that cools, slip the 4 rings of the blades onto the newly-made spool. Make sure all the blades are facing the same direction.
Make another circular ring out of plastic and 3mm craft foam (the same dimensions as the first one, of course,) and glue it to the other end of the spool. Since you'll have so little space to work with at this point, you might wind up hotgluing the blade at that edge of the spool to the inner side of the circular ring. That's okay. As long as the other blades move around freely, you'll be okay.
And this is what you should have. Here's what the shuriken looks like in it's closed position.
And here's how it appears when it's been unfolded. Now all you have to do is paint it. (Yaaaay...)
I began painting by unfolding the shuriken and spraying it on both sides with Testor's black spray paint. Once it had completely dried, I was able to work on the fine detail painting, where I mixed glossy black with a little silver and dabbed it over the surface of the blade to create a mottled, metallic look. For the silver edge, I applied pure silver metallic enamel in thick short strokes to create the illusion of razor-sharpness.
Here's what the shuriken looks like when it's closed.
And a full shot of the shuriken when it's open...
I know I played fast and loose with the explanations, but hopefully you should be able to get a good idea of what to do from the pictures. And as always, if you have any questions or comments, e-mail me and I'll get back to you as soon as I can.