Step 1: Logon to https://manage.windowsazure.com (old portal) because this featurehas not yet migrated to the new one.
Step 2: Select SQL databases from left side panel and select a database which you need to export.
Step 3: Select Configuration tab at the top.
Step 4: Select export status to automatic instead of manual.
Step 5: Select storage account, where you want to store the backups.
Step 6: Select Frequency to how often to take the backup of selected database.
Step 7: Select Start Date to set when it should start taking backup.
Step 8: Select retention days to how many days the particular database backup should be retained or kept in the storage.
Step 9: Add server login credentials.
Step 10: Save the settings and it will start taking backup as per the scheduled time and given frequency.