As cortex itself is only a Rails Engine, it needs to be mounted within a parent Rails applicaton. cortex-starter serves as a starting point for new users, with cortex and cortex-plugins-core already mounted and configured with several example ContentTypes/Decorators. Start by cloning the repository:
Ubuntu and Redhat/Fedora do not have an official elasticsearch package - you must use Elasticsearch's repositories for APT or RPM or follow these manual instructions. The same goes for phantomjs. Build from source or use a PPA. Additionally, yarn will need its repository added for these distributions. Follow these instructions. Other Linux distributions likely have these as prebuilt packages in their official or user repositories.