Did this in a bathroom many years ago after taking down 1947-era tile. Needless to say you were not going to sand or even chip off that old glue over plaster. I coated with ordinary drywall mud, at least a couple of coats and sanded it down. Should say I used to be a drywall finisher so it wasn't a big problem for me, but is still a job. But it can be done successfully. Good luck.