Dan Frakes:

I’ve long recommended creating a bootable installer drive—on an external hard drive, thumb drive, or USB stick—for the version of OS X you’re running on your Mac. It’s great for installing the OS on multiple Macs, because you don’t have to download the ~5GB installer onto each computer, and it serves as a handy emergency disk if your Mac is experiencing problems.

Dan takes you through all the steps using Terminal.