Set up your Illustrator layers
Keep your full-color design on a CMYK Art layer. Create a separate White Ink (Spot1) layer above CMYK Art and place the shapes that need white ink there. Turn on Overprint Fill for the white-ink objects.
Prepare Your File for White Ink
Printing on clear film? Set up a separate white-ink layer and Spot1 swatch so white ink can print beneath your full-color artwork.
Create the Spot1 swatch
In Window → Swatches, create a swatch named Spot1 and set its type to Spot Color. Apply it to the white-ink objects. Its display color is only a visual indicator in Illustrator.
Save for white-ink printing
Export a PDF with the Spot1 color separation preserved. When printed on clear film, the white underbase prints behind the CMYK artwork to make the colors appear opaque.