You choose the modes as you cast the spell. Once modes are chosen, they can't be changed.
2023-10-13 • wotc
No matter which combination of modes you choose, you always follow the instructions in the order they are written.
2023-10-13 • wotc
No player can cast spells or activate abilities in between the modes of a resolving spell.
2023-10-13 • wotc
If an effect copies Doomsday Confluence, the copy will have the same modes chosen the same number of times.
2023-10-13 • wotc