offers excellent walkthroughs on the manual installation process. Important Compatibility Check
softwareupdate --fetch-full-installer --full-installer-version 10.13.6 Download Macos High Sierra 10.12 Dmg
which will redirect you to the App Store for a secure download. For macOS Sierra (10.12): Apple provides a direct DMG download for Sierra 10.12 Download Macos High Sierra 10.12 Dmg