Hi - so there are a couple of ways you can sequence events in a nonfiction narrative. You can use the reporter's formulas of Who, What, When, Where, Why, and How which details the essential facts of the history in this case of the Rosetta Stone. You could combine it with another outline I use for narrative writing that may help get those key events into a timeline of sorts.
then the next moment
It kind of mirrors a timeline of sorts. With this, you need to set up a bit of background of why this was such an important find. Why was it important in the world at the time? How did it change the world after it's discovery and such. I'd add important names and places, but not every name or place if they are just fillers.