Yup, smooth side down. Lowes sells a 10X3 in glass tile for about 5 bucks that once lapped flat, with a diamond plate, (quick & easy to do) makes a great substrate for film.

You will see the high spots.

A piece of copy paper under 1um film makes a great finish, polishes the edge of a 1um edge. 1um over paper is a very comfortable edge.

Diamond film is not 10 times better than Aluminum Oxide film, in fact not a better finish at all, certainly not justifying 10X the cost.