Gluing sheets of abrasive paper to plywood provides a flat surface to sand on. I have been using the grey paper that tends to release the sanded material quite well.
I have been slightly thinning some PVA with a little water and brushing onto both the ply and the abrasive before sticking together and placing weights to keep everything flat.
On the back I drive some panel pins about half way through the ply.
Nip of with pincers so the cut pin protrudes, When placing the sanding board on the workbench you just bang the 4 corners by hand to get the pins to grab, this hold the panel firm on the table for sanding.