From red99 includes where to get material: http://wlsheadliners.com/
I just replaced the headliner in a 1998 Bug a few weeks ago. A much lengthier (thought not very hard ) job.
The process was as follows once you have the headliner removed;
Pull the existing headliner covering off of the shell. This will leave the foam backing on some or all of the shell.
Take a clean and dry scrub brush and brush off as much of the foam as possible. I was able to get 98% of it off the Bugs headliner.
Layout and rough cut the new headliner.
Pull back the rear half of the new headliner cover and spray half of the headliner and new covering. (3M or DAP contact adhesive spray, I'll cover this at the end.)
Wait ten minutes then butt the back of the coverings to the back of the headliner shell. Start from the back and smooth the headliner in place with a firm rubbing with your hands. The R170 shell is an easier run than the Bugs so it should be an easy shot.
Once you have half done repeat for the front half but picking up where you left off on the headliner. Be sure you have all of both surfaces covered.
Once complete let it sit for a few then trim it up nice and neat then install the new headliner.
My daughters Bug headliner looks like new.
Where to get the material as a kit and it includes the spray?
The kit for the bug was about $60 total.