Simple answer: you don't. Longer answer: It depends. Do you want to use yum or apt-get to install programs? If you like CentOS, create a CentOS container with Docker and begin installing programs using yum! Ubuntu? Spool up an Ubuntu container and apt-get away! CoreOS is just the cradle upon which you can run the userland of any distribution you please.
So CoreOS is the simple host OS, thank you for the clarification, i thought it was a minimal guest OS.
I believe it's intended to be a minimal OS for running containers like those made by Docker (http://www.docker.io/).