Lee Nowell expressed precisely :
I am installing a new shower tray and the waste it comes with has a very thin
washer that goes on top of the tray and underneath the waste fitting. Is this
enough to seal the top or should I silicone it on to the tray? Or I guess fit
it and silicone around it?

Put some Plumbers Mait around the sealing points, plus long the threads
too, to be sure of no leaks. It's none setting, so comes off easily
where it squeezes out or if it needs to be disassembled later.