Backing up your Mac is really easy using Time Machine. It is free. All you need a large portal hard drive or USB. You can even set a reminder to encourage you to back up your files often. Just remember that USB and hard drives do not last for ever.. Make sure your drive is large enough, fast enough (USB 3) and Mac compatible.
Visit apple.com click support and search Back up your Mac with Time Machine for most lasted directions from Apple.
As of April 19 2021 these are the directions given by apple support
"Use Time Machine, the built-in backup feature of your Mac, to automatically back up your personal data, including apps, music, photos, email, and documents. Having a backup allows you to recover files that were deleted, or that were lost because the hard disk (or SSD) in your Mac needed to be erased or replaced.
Create a Time Machine backup
To create backups with Time Machine, all you need is an external storage device. After you connect the storage device and select it as your backup disk, Time Machine automatically makes hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for all previous months. The oldest backups are deleted when your backup disk is full.
Connect an external storage device
Connect one of the following external storage devices, sold separately.
· External drive connected to your Mac, such as a USB or Thunderbolt drive
· Network-attached storage (NAS) device that supports Time Machine over SMB
· Mac shared as a Time Machine backup destination
· AirPort Time Capsule, or external drive connected to an AirPort Time capsule or AirPort Extreme Base Station (802.11ac)
Select your storage device as the backup disk
1. Open Time Machine preferences from the Time Machine menu in the menu bar. Or choose Apple menu > System Preferences, then click Time Machine.
2. Click Select Backup Disk.
3. Enjoy the convenience of automatic backups
4. After you select a backup disk, Time Machine immediately begins making periodic backups—automatically and without further action by you. The first backup may take a long time, but you can continue using your Mac while a backup is underway. Time Machine backs up only the files that changed since the previous backup, so future backups will be faster.
5. To start a backup manually, choose Back Up Now from the Time Machine menu in the menu bar. Use the same menu to check the status of a backup or skip a backup in progress."
Some additional suggestions from our staff at McClanahan Camera for after you backup the device
make sure you label the backup with date, and device backed up
put this copy in a safe place
consider making another copy and keeping it off site or at another location in case of fire at your home or office or just plain misplacing the storage device.