Download Mac Os Sierra 10.12 Dmg -

Once you have the Sierra DMG or installer app:

sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/SierraUSB --applicationpath /Applications/Install\ macOS\ Sierra.app --nointeraction

For best performance on old hardware, perform a clean install. Download Mac Os Sierra 10.12 Dmg

Restart your Mac and hold Command (⌘) + R or Option (⌥) on startup.

Select the USB drive as the startup disk.

In Disk Utility, erase your internal drive (Format: Mac OS Extended). Name it "Macintosh HD".

Quit Disk Utility and select Install macOS Sierra.

Warning: Be extremely cautious with third-party websites. Many "free DMG download" sites inject malware or modified installers. Always verify checksums. Once you have the Sierra DMG or installer app:

Use this official Apple link on a Mac running High Sierra (10.13) or earlier: sudo /Applications/Install\ macOS\ Sierra

https://apps.apple.com/us/app/macos-sierra/id1127487414?ls=1&mt=12

Note: If you are on a modern macOS (Mojave+), the link may redirect. Use a compatible Mac or the terminal method below.

If the official links are dead, use these verified archival sources.