start new:
tmux
start new with session name:
tmux new -s myname
These instructions install ruby and rubygems from source, not from RPMs. For instructions on installing from RPMs, look at something like http://wiki.opscode.com/display/chef/Installation+on+RHEL+and+CentOS+5+with+RPMs or http://mykestubbs.com/blog/2010/03/chef-installation-on-centos-54.html