how to downgrade from Mojave

You could use your Time Machine backup to revert to a time before you did the upgrade. But you would not have any files you created or updated after installing Mojave.

The other method would be to back up your existing files/foldlers, download High Sierra and put the installer on a separate hard drive. Then erase your hard drive, reinstall High Sierra and replace your data.

Get macOS High Sierra.

John Galt wrote:

I would point out those files will remain available to restore after restoring the earlier backup. In other words restore a High Sierra system, and then restore whatever individual files and folders that were subsequently backed up.

Yes, of course. More coffee needed!

macjack wrote:

... But you would not have any files you created or updated after installing Mojave.

I would point out those files will remain available to restore after restoring the earlier backup. In other words restore a High Sierra system, and then restore whatever individual files and folders that were subsequently backed up.

Temporarily disable Time Machine's automatic backups to preclude any possibility of another scheduled backup requiring the deletion of old, "expired" backups that might occur, should TM require the space they occupy. You'd have to be pretty unlucky for that to happen but why tempt fate.

To learn how to restore a previous macOS version please read Restore your Mac from a backup - Apple Support.