How can I glue this material to a flat surface, probably ply ? It has a glossy plastic backing so I assume a glue such as pva will not work; I intend to use the film for sharpening as it has a 20 micron grit.
Just to second what Droogs said - get some plate glass, or at least MDF. Ply wont be flat enough for sharpening. If you do obtain glass then if spray adhesive isn't to hand you'll find that water alone will eventually achieve enough grip between the 3m backing and the glass. You just need to use a finger to hold the 3m in place to get you started and then suction will grip enough.
All the films I've used are self adhesive so your shiny side should peel off. Some of the older films have paper backing but again these should be self adhesive.
When it comes to replacing films, getting the old adhesive off your flat surface can be a problem but acetone (as in nail varnish remover) works well - tip courtesy of Peter Sefton.