When it's used for dialog, it's usually stored like any other dialog - first character displayed first, last character displayed last. The tricky bit comes with reprogramming the game to display left-to-right text roman-style, which is an advanced topic you're probably not ready for.
When it's used for the occasional one-off stillscreen, you still have to figure out how the game is rendering the screen, but past that it's usually like any other graphic. You rewire the tilemap/pattern table/whatever the same way you'd do any other. A title screen, for instance.