Removed the headliner from 2004 Silverado. Single cab. I removed all the interior trim and sunvisors, grab handle, dome lamp and coat hooks.
Removed the third brake light on the rear roof to get at the snap in attaching to release the head liner backing board.
I peeled the cloth from the foam that was glued to the fiberglass headliner board.
I proceeded to remove the foam to get a clean surface for the new material.
I started with sandpaper but that was slow going; various grits and sanding blocks...not really effective.
I tried putty knives in a variety of widths, dragging them upright like a paint scraper, but not really effective.
Then I tried a belt sander very uneven results and to much dust!
Finally tried a wire brush....

Very much better!
One hour later:

Had one mishap:
Next up will be attach new material.
I purchased 60"X40" of headliner material from the Springfield Wal Mart store fabric department. Color selection was Dark Grey, Beige and White; foam backed. Also, can of spray Upholstery Adhesive.
$21 altogether...