Connect the external hard drive to your computer
Make sure the external drive has enough free space
Connect your iPhone to the computer with a USB cable
Open Finder on Mac or iTunes on Windows
Select your iPhone
Choose Back Up Now to create a backup on the computer
Locate the backup folder on your computer
Copy the iPhone backup folder to the external hard drive
On Mac, the backup is usually in `~/Library/Application Support/MobileSync/Backup/`
On Windows, the backup is usually in `%APPDATA%Apple ComputerMobileSyncBackup`
Paste the backup folder onto the external hard drive
Keep the external hard drive safely stored
Repeat the backup process regularly