Mac OS X Repair Service Bootable USB Flash Drive Install High SierraPerfect for older Macs.
flash drive USB 3.0 so you can install fresh, wipe system, restore, upgrade
you can check if your Mac's hardware is compatible here
https://www.macworld.com/article/673697/what-version-of-macos-can-my-mac-run.html
1.Insert the flash drive2.restart computer upon restart before the chime hold down the option key (you might have to try a couple of times)this will take you to the booting screen3.using the cursor keys choose the operating system then hit enter4.once in the installer you can install from there by following on screen instructions