How do I set up a white mask file?

1) Finish your full-color artwork. 2) Duplicate it so the mask is the exact same size, with the same bleed. 3) Fill every area that should get white ink with 100% black (C0 M0 Y0 K100). 4) Make everything else white. 5) Save the mask as a separate vector PDF or EPS that lines up exactly with your artwork.