In this video I explain how to modify two circular polarizing filters to turn them into a working Fader ND Filter. Its very simple, buy two cheap polarizing filters that fit your camera.
take one put it on the camera then take the other and removes the glass lens and flip it inside out, put it back in the lens ring and put it on top of the other filter already on the camera. the modified filter must go on the outside. when you turn the outside ring you will see the filter increase or decrease in darkness.

DV Rebellion