Start out with squares of paper, cut to about 6x6". I recommend using double sided scrapbook paper, though, of course, any paper will work.
Fold your paper in half once to make a triangle.
Fold this triangle in half to make a smaller one.
Open the square back up, and cut about 3/4 of the way up each crease, toward the center.
Fold one corner from each of the four resulting triangles up to the center of the square. (Ignore the tack in the middle, as it is only there at this step in order to take the picture.)
Overlap the four corners and pierce them all with a pushpin. Finish up by pushing the pin into a wooden dowel.
Bonus: Follow the above directions, but glue the corners together instead of using a pushpin. Glue a button in the center and then glue your pinwheel to a mini-popsicle stick. These make amazingly cute cupcake toppers!