Mac Technique: Steps to Create a Bootable macOS Installer on a USB drive

This post shows how to create a bootable USB drive for macOS Sierra installation. The USB drive was created with a MacBook Pro running OS X 10.8.5 (Mountain Lion), which was installed on a clean SSD by holding down the option key during the first boot after swapping the Mac's hard drive with the SSD.

The steps to create the bootable macOS USB installer are as below:

The original OS X version is Mountain Lion 10.8.5.

Go to the Mac's App Store and get the latest macOS.

Quit the macOS installer.

The installer may be found in the Applications folder of Finder:
Application\Install macOS Sierra

Open the terminal and type this command:

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

For other paths for USB drive or  Sierra.app or for installing other OS X versions, refer to Apple's official document: Create a bootable installer for macOS.

Now the USB drive has been successfully formatted as a bootable macOS installer.

Reference:

Create a bootable installer for macOS