How do I back up High Sierra to an external hard drive

How to use Time Machine to Backup or Restore your Mac:

https://support.apple.com/en-us/ht201250

If you choose to connect your Time Machine backup drive only from time-to-time, try to do so at least every-other day, at minimum. Otherwise, it may take several hours of computation just to decide what needs to be backed up, before any files are transferred.

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Leroydouglas also wrote a nice article about Boot Clones. This might be something to think about a little further down the line.

 How to create a boot clone

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I keep both a Time Machine backup and clones. You can use a cloning software like SuperDuper! or CarbonCopy Cloner.

The advantage of the clone is that you can boot from it and use the system. So, if your hard drive fails you can just boot from the backup and keep on working until you have the time to repair or replace the internal drive. You can also drag and drop files with the clone because it is the identical file structure on your primary drive.

The advantage of TM is that it creates recursive backups, so you can restore a file or the entire drive from a certain point in time.