How do you schedule when a backup occurs

iCloud backups occur once every 24 hours, when the device is screen locked, plugged in and connected to wifi. The only way to try and set when a backup will occur is to manually make one at the time you want to set the 24 hour timer.

Note that whenever the device cannot backup to iCloud at the 24 hour mark, it will try again the next time it is plugged in, screen locked and connected to WiFi. So that would then reset the 24 hour timer for the next backup.