Creating a OS Sierra USB Install Drive Using the Terminal

If you don’t want to download an extra bit of software, you can make your own installer USB drive using Terminal.

• Download the OSX installer macOS Sierra (in this example which is in Applications folder)
• Insert an 8GB (or larger) flash drive and give it a name. For this tutorial, we’ll use the name Untitled. Make sure the drive is formatted for OS X Extended (Journaled). If it isn’t, open up Disk Utility and format is so it is. Before you do so, back up any important data on that drive. It’s best to disconnect any other external hard drives or flash drives so you don’t mix them up.
• Open up Terminal (Applications > Utilities).
• Type (or copy and paste) this command into Terminal, replacing Untitled with the name of your drive, then press Enter:

sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Sierra.app --nointeraction &&say Done

• Type in your password when prompted and press Enter.
• Let the command line do its work and don’t interrupt it until you see the final line that says "Done". This can take a while, so be patient.
• When it’s done, insert your USB drive into any Mac, then launch the installer by holding down the Option key when you boot up your computer.

Source (http://lifehacker.com/how-to-make-a-bootable-macos-sierra-usb-flash-driv...)