Step 4: Install macOS High Sierra via USB. Once you have your bootable macOS High Sierra USB installer, you can use it to install High Sierra on compatible Macs in one of two ways.

OS X and macOS have had the capability to create install media for quite a while, but this isn't widely known, for two reasons. First, the command to create the bootable installer is well hidden within the installer that's downloaded from the Mac App Store; and secondly, the installer you download has a really annoying habit of automatically starting up once the download is complete. If you then click the install button, you'll find that the installer you downloaded is automatically deleted as part of the normal installation process, preventing you from using it to create a bootable macOS Sierra installer of your own. • A copy of the macOS Sierra installer, available from the Mac App Store. It sometimes takes Apple a little bit of time before the Mac App Store is updated with new releases such as macOS Sierra.

If you don't see the operating system available in the store on day one, try performing a search for macOS Sierra in the store. The macOS Sierra installer will start up automatically once the download is completed. Do not continue with the installation as prompted by the installer; instead quit the installer.