That would depend on the structure of the government. I would assume that under her, (unless it was a minuscule empire and she personally managed things like rubbish collection,)there would be various heads of different areas, let's call them commonwealth heads of state, and she would call them together and tell them and they would then plan the possible transitions, including timescales and the various elections and handings over of powers, then they would plan the decision making process (referendum, government vote, toss a coin) the ceremonial activities and timescale, the announcements to the outside world, also to the people of the empire telling them what would happen and how it would happen. There would also have to be duscussion on hs to divvy up resources, the splitting up of such offices a tax, army, security, borders, any mineral wealth, airspace, common travel structures, banks and currency, law and the judiciary and so on, also new mechanisms for the individual parts of the old empire to talk together....
A big subject....are you writing a book?