Press the to turn on the Mac, and immediately press and hold the Option (Alt) key. Release the key when you see the Startup Manager screen.
Step 4: Create a Bootable USB Drive (Clean Installation Method)
To perform a clean install, you must create a bootable USB drive using the createinstallmedia command in Terminal. Plug in a USB drive (16GB+ recommended). download macos sierra 1012 dmg file install
Step 1 — Obtain the macOS Sierra (10.12) installer or DMG
Disconnect your Mac from the internet (unplug Ethernet and disconnect Wi-Fi). Press the to turn on the Mac, and
: Restart your Mac while holding the Option (Alt) key. Select the USB installer from the boot menu and follow the instructions to install.
Press , type your system administrator password when prompted, and press Enter again. Plug in a USB drive (16GB+ recommended)
: Open Terminal and run the following command to move the installer files to the USB:
If you just miss the design, consider running Sierra in a virtual machine (VMware or VirtualBox) instead of on bare metal.
Scroll to the section for older versions and select the macOS Sierra 10.12 link.