Connect your iPhone to your desktop. Use the USB cable that came with your device.
If this is the first time that you've connected the phone to your computer, you'll need to tap "Trust" on the iPhone's screen.
Click on your iPhone icon. It will appear after a moment in the row of icons along the top of the window.
This opens the "Summary" screen.
Unlock your iPhone. If your iPhone is currently locked with a passcode, it will need to be unlocked before you can start the backup process.
Click on "This computer" in the Backups section. This enables iTunes to back your iPhone up on your computer, saving your iCloud storage space. A backup will also be created during a sync process.
If you wish to back up your passwords, Homekit data, or Health and Activity data, you will need to check the box that says "Encrypt iPhone backup" and create a memorable password.
Click On Back Up Now. This begins the backup process.
You may be prompted to back up apps that aren't currently in your iTunes library. This can happen if you installed apps from other sources or haven't transferred your purchases from your iPhone to your iTunes library. You won't be able to restore these apps unless they've been added to your library.
You may also be prompted to transfer purchases from your iPhone to your iTunes library. This will occur when you have installed apps on your iPhone but don't have iTune set to automatically download new purchases.
Wait for the backup process to complete. Once you have confirmed whether or not you want to backup apps and transfer purchases, your iPhone will begin backing up to your computer. You can monitor the progress at the top of the iTunes window.
iTunes will backup your settings, contacts, app data, messages, and Camera Roll photos. It will not backup any synced music, videos, or podcasts from your iTunes library, or any other media added through other means. These will need to be re-synced using iTunes after the restore process.
The iPhone backup file is saved to your iTunes Media folder.