PLEASE HELP: Print Illustrator file to CMYK separations w/ underbase for screenprinting on blacks.

If you have a vector file are you sure a CMYK sep is what you want? are there more than 4 colors in the Ai file? If not then spot color would be easier & cheaper.

Is there photographic material in the file or more than 4 colors?

If so you would probably actually have an easier time exporting a 300 dpi image from the Ai file & doing the CMYK sep in Photoshop. Easier to do.

Here's a good article to read...

https://t-biznetwork.com/blogs/scottfresener/secrets-to-high-end-printing-on-dark-shirts-by-scott-fresener/

I did wholesale screen printing for 27 years and used Fastfilms for the "simulated" four color process printing on black t-shirts with fabulous results. 

Here is an adobe help document to read through.

https://helpx.adobe.com/illustrator/using/printing-color-separations.html

If your artwork needs to be "White" for screenprinting ink, then you can create a brand new spot color in Illustrator, and name the color "White".