You can easily create a Mac OS Sierra bootable installer on a USB flash drive using Terminal. These steps will walk you through the process. (Other OS X versions click here.)
Steps to Create a Mac OS Sierra Bootable Installer
Get the latest installer
Download the latest macOS Sierra Installer from the AppStore. If you get a pop up to install Sierra, simply ignore it or force quit the install from the Apple menu. Next find a USB flash drive that is at least 8GB or larger. Back up any files you want to keep off the drive as it will be erased. Insert it into one of the USB ports on your mac. This flash drive will be used for the create a Mac OS Sierra bootable installer.
Use the Terminal app
Use the following example. This is the basic syntax of the command. Replace
volumepath with the path to your USB flash drive or other volume, and replace the installer path with the path to the Install macOS Sierra app. In many cases this will be in your Applications folder. The flash drive called “MyVolume” will be completely overwritten. Change this name with the name of your USB Drive.
Copy and paste the following example into Notes or TextEdit. Then change the name of “MyVolume” to the name of your USB Drive.
sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia –volume /Volumes/MyVolume –applicationpath /Applications/Install\ macOS\ Sierra.app
After replacing “MyVolume” copy and paste the entire command into the Terminal app. Hit return. You will be prompted to enter your password. Note: You must be an administrative user on the Mac in order to run this command.
Monitor progress in Terminal
Next you will be prompted to enter “Y” to proceed. Then the process will begin. Read the output information displayed in terminal to follow the progress. It will erase your USB drive, rename your flash drive, and then copy the installer files. Once it is finished, you will be returned to you may quit Terminal. Be sure to eject your macOS Installer.
Using the Installer
To use the installer, turn off the Mac you want to upgrade. Insert the flash drive. Press the power button while holding down the Option key. Continue to hold “Option” until you see the start up disk options. Select Install macOS Sierra.
Find out more from Apple’s KB here.