The order matters more than anything else, and the rule is: build the export before you announce.
Announcing first produces a wave of people asking how to get their data out, discovering there is no good way, and being angry at exactly the moment you have no leverage and no time. Ship the export, test it, then tell people.
A reasonable sequence:
- Build and test a complete export. Everything they put in, in a format they can actually use: not a database dump. Open formats, one download, and check it by importing it somewhere yourself.
- Stop new signups and stop charging. Nobody should be paying for a service with a known end date, and turning off billing early buys you enormous goodwill for very little money.
- Announce, with a date, and say plainly what happens and when.
- Remind them. At least twice more before the date. People ignore the first email.
- Keep it read-only for a period after the end, if you can afford to.
- Then turn it off and delete.
The read-only period is the cheapest kindness available, the running cost with nobody writing is usually small, and it catches everybody who missed the emails.