It is a all-in-one ND FILTER you just turn the front ring and the unit darkens to whatever setting you desire, it fits on the front of the camera, but, you must remove the screwed on filter that is supplied as a standard with the Phantom 3 advanced/pro.
If you want a cheap Polarizing Filter, I used a LEE filters swatch, cut to the size of the screw on ring,
It is roughly the size of a 2p piece.
If you cut a very small "moon shape" at the top on the edge (when the filter is at its darkest) that will make aligning it when the screw lens is put back on the front of the camera (the polarizing filter is put on the inside of the existing filter ring, but make sure it's not to tight, or you will not come out to easily !!).
Both give great results !