You're going to need the unique ID, which has been assigned to an RDS read-replica. If you followed the previous Creating a database with automatic failover, and Creating a database read-replica recipes, then you'll be all set.
It's also a good idea to have backups enabled on this read-replica prior to promoting it. This shortens the promotion process because you won't need to wait for a backup to be taken. You'll want to set the backup retention period to a value between 1 and 8.
Enabling backups on your read-replica will cause it to reboot!
In order to enable backups, you can use the following CLI command:
aws rds modify-db-instance \ --db-instance-identifier <identifier-for-read-replica> \ --backup-retention-period <days-to-keep-backups-for> ...
