First things first, you don't want a screen larger than 6" as it'll be too big to fit comfortably in a Passat headrest. I fitted 7" ones on my old Passat and they stick out of the edges and just look wrong. The ones I have chosen are Veba 5.8" screens and they are also fairly thin at 20mm which will help them fit flusher in the headrest.
First off, place your screen in it's frame on the headrest and decide where you want it to go. Make sure you place it on the back of the headrest!
Then mark the four corners on the headrest and draw a line between them to form a cross
Next, using a sharp blade and a ruler, cut along the lines and peel back the flaps of cloth and pin them back to reveal the rectangular shape your screen is going to go in.
Using your knife, cut out the foam, going down as deep as you can until you hit the plastic body of the headrest. Cut the hole smaller than you need at first in width and length, then place your screen on and when you're happy with position of it, cut the rest of the hole out to the right size. It's a lot easier to cut more foam out than stick it back in.
When you've cut you hole and you're happy the screen fits on there snuggly, unclip the screen from its frame and place it into the hole and look for gaps around the sides where the headrest curves at an angle and makes the frame seem to stick out. Cut strips of the removed foam and use them to place under the headrest cloth around the edges of the frame to bring them up flush together. Once happy, tuck the flaps of material under the headrest foam to secure the there.
When you're happy with the fit, place your frame in and drill four small holes through the plastic of the headrest at the frame mounting points and use small self tappers to secure the frame to the headrest. Before you secure it down though, thread the short cable from the screen through the hole in the bottom of the frame and through the bottom of the headrest to come out by the pole. It will be a lot easier to do this before the mounts screwed down.
The screen then simply clicks into place and you're ready to go.
When you've done your screens, run your cables through the seats and under the interior trim (a coat hanger and gaffer tape works well to thread the cable through the seat) and wire it up to your head unit and you're done. I've gone into greater detail on how to wire it up in my previous 'How To' on fitting headrest screens so I'll leave that there.
Any questions, just ask.
Giles