An 8GB+ USB drive.
Once you successfully install 10.13.6, immediately use SuperDuper or Carbon Copy Cloner to make a bootable backup. As Apple scrubs old installers from its servers, finding a working DMG will only become harder. Macos High Sierra 10.13.6 Dmg
Apple no longer provides iCloud Keychain verification or two-factor authentication prompts reliably on 10.13.6 after 2023. You may need to generate app-specific passwords for iCloud. An 8GB+ USB drive