Reader messages are the little sentences that pop up from the top of your screen.
This is what you would put into your script to have the readermessage pop up on your screen.
INT/EXT. THE SCENE - DAY/NIGHT
readerMessage (your sentence)
INT. BLACK - DAY
readerMessage A few years ago…
It’s very simple and easy to use. There is nothing to really explain although, I would watch a tutorial on YouTube or put a form of dialogue before saying readerMessage.