Is the metal dash frame out of the car? If so, you'll find it a lot easier to work on it. Also easier if the glass is out. The order that worked best for me was 1) refurbish the face material, 2) install new upper dash foam and vinyl, tucking the excess around the curved cutout for the vents, 3) Install the new dash top (protecting the upper dash from scuffing by the dash top studs). 4) Install the entire dash as one unit into the car. If you have a helper, it'll be easier to place the wiring harness in the right spot when the dash goes in.
When I did mine, the windshield was out. I took the defroster vents off by removing the spring clamps that hold the two halves together in the front trunk (they're very fragile, be careful). Once the pad was in place, i gently worked the vents back up into the slots. Then finished it off with a new rubber dash-to-windshield seal from 914rubber.
Here's a pic. Enjoy!
Click to view attachment