Create a High Sierra Macintosh OS installer

On a Macintosh computer, insert a thumbdrive and make sure it it formatted to MacOS extended Journaled

Open the App store and search for High Sierra.

Click 'Download' and wait for the OS to download

Once it has finished downloading, it will launch, you will need to close out of the installer.

Open 'Terminal'

type this command in terminal 'sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/{ThumbDriveVolumeName}'

It will then begin the process of converting your USB drive into a bootable High Sierra installer. Please make sure that you replace {thumbdrivevolumename} with the name of your actual thumbdrive.