Task: I want to add the text "We can't help everyone, but everyone can help someone" to the Flash card presentation's first slide in the Google Slides app.
Steps:
click on the plus icon (add) at the top of the screen to add text in slide 
choose text option (top second  button) at the  bottom of the screen from pop up window 
type We can't help everyone, but everyone can help someone" in the center of the slide  screen