1: Where do you get this “door skins” material?
If you are in a country such as Australia, you probably cannot. In the USA/Canada it seems very easy to get, often given away, it seems.
Since that wasn’t going to happen for me I went out and bought a 40 y.o. veneered woodgrain looking complete door for $10. I was warned that it would be difficult to reclaim the ply and in fact it was. But not impossible. Just cut the internal timber framing away including the door handle area. Depending on how old the door is, cut and pull the glued paper honeycomb away leaving a tiny but slightly raised thin contours of glue behind. For me I have a belt sander so with a rather worn coarse grit belt fitted (so it didn’t devour the thing!) I easily removed the residual glue leaving a relatively flat surface.
But despite the nicety of having a permanent timber veneer looking deck, etc, it is probably not worth the bother.
Instead I found that you can get 3mm pine ply pretty well anywhere, premium quality has a smoother, unblemished finish (check for the best looking piece) and at around $12 (down here) for a 1200 x 800 sheet, oodles for the job, and extra if you need it (see later). In Oz you can get it at Bunnings [Click on Link]