My method is to use double-sided 3M sellotape. Cut it to fit the plate, ideally just a fraction smaller. Sit the plate on a smal blob of Blu-Tak. Attach to the plate, then peel off the paper covering the outer sticky layer. Pick up the Blu-Tak with tweezers, press firmly onto the loco side, hold the plate down with tweezers as you remove the Blu-Tak off it. It's never failed me over the years.
RWJ