:Inside the mounted disk image, you will find a file named InstallMacOSX.pkg . Double-click this to begin a brief installation process.

Before installing, verify your Mac meets these minimum technical specifications: How to download and install macOS - Apple Support

sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/YosemiteUSB --applicationpath /Applications/Install\ OS\ X\ Yosemite.app --nointeraction

: Always perform a full backup using Time Machine before any OS installation.

2 GB Required free storage: 8 GB for the installer + ~12 GB for the OS itself.