Connect your iPhone or iPad to a PC or Mac with iTunes installed. Now click on the icon at the top of the screen that corresponds to your device. This takes you to the Summary screen.

Under Backups, click the Back Up Now button to kick off the process.

You'll now be asked if you want to encrypt the backup. If you don't encrypt the backup then sensitive information won't be stored in the and in the event that you have to restore your device, some data will be lost.

Since you want to be safe, click Encrypt Backups.

You'll now be given the option to choose a password. Make it a good one!

If you store the password in the Keychain (a Mac-only option) then you won't have to remember the password later. The upside of this in convenience, but the downside is that your password will be stored on the mac, and if something happens to the keychain, your backup is history.

Click Set Password to begin the backup process.

Backing up will happen in the background. When it's done you can verify that the backup has completed successfully by going to Preferences and clicking on the Devices option.

The lock icon tells you that the backup is encrypted.