you can print out the pattern here. Cut along the solid lines only. Fold on the dotted lines.
Fold A toward you. Fold B away from you.
Fold C and D over each other so they overlap.
Fold the bottom up and put a paper clip on it.
Hold the Roto-Copter by the paper clip. Throw it like a baseball, as high and far as you can. It will spin to the floor. You can also stand on a chair or on the stairs and drop it.
thanks to Science Explorer for this wonderful gadget.