NOTE: Although Apple has updated its information, the following instructions will not work unless you have the second version of the Install macOS High Sierra app installed. If you have the first version, which was only 19.9 MB, you will need to trash that, and download the revised installer which is about 5.2 GB.

One supported method of installing High Sierra which is ideal when you want to format its startup disk, or want to install it on multiple Macs, is to make a bootable installer on an external drive, and install from that. Soon after the release of High Sierra, third-party tools should become available to make this even simpler, but for the moment the following is Apple’s recommended method.

Initialise the external drive, such as a USB memory drive. If you’re going to be using the drive to upgrade Macs from earlier versions of macOS, don’t initialise it using the new APFS file system, but stick to a more traditional format. You may find this easier to do in Sierra, before upgrading.

If you’re only going to be using the bootable installer with systems which have already been upgraded to High Sierra, then you can format it in APFS if it is an SSD or Flash drive. Ensure that you do this with a Mac which is already running the release version of High Sierra, or use the version of Disk Utility which is supplied in the latest High Sierra Install Assistant from the App Store.

The external drive must have a minimum of 10 GB free space.

If you don’t already have a copy, download the latest Install macOS High Sierra app from the App Store.

Open Terminal, and type the command
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/[volname]
where [volname] is the volume name of the external drive which is going to be your bootable installer. If you’re unsure, you can always type the command
ls /Volumes
to see a list of all mounted volumes. Typing
/Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia
returns usage info about the command createinstallmedia

Once the createInstallMedia command completes, your bootable installer is ready to use.

if you’re successful in creating one, to use your bootable installer, connect it to a target Mac. You can then either

• open the Startup Disk pane, select the bootable installer, and restart the Mac;
• restart the Mac with the Option key held to enter the Startup Manager and select the bootable installer, or with the C key held to boot from a USB memory drive.

Apple’s full instructions, which cover earlier versions of macOS / OS X too, are in this article, and finally give information for High Sierra.

(Amended 26 September 2017 following High Sierra release, to reflect correct information.)