This is the safest method. You will receive the "Install macOS High Sierra.app" in your Applications folder, which you can then use to create your own bootable DMG.
If your Mac is too new to download High Sierra from the App Store, you can use a utility called Mist (GitHub) . download macos high sierra 1013 6 dmg file link
macOS High Sierra 10.13.6 full installer - Apple Communities This is the safest method