: Locate the downloaded InstallMacOSX.dmg in your Downloads folder and double-click it to mount the image.
If you manage to obtain the installesd.dmg or a similar installer package, you can create a bootable USB drive with it. However, if you only have access to a newer version of macOS, you might need to use different commands to create a bootable installer. mac lion os x 10.7 - installesd.dmg download
A Mac running a compatible OS (like High Sierra or older) to use Disk Utility. Plug in your USB drive and open Disk Utility . : Locate the downloaded InstallMacOSX