You might run into the problem where Yum says that there are no packages for Nagios.  If that is the case then add this to your yum repos.  For CentOS it was called /etc/yum.repos.d/CentOS-Base.repo.

# cat /etc/yum.repos.d/linadd.repo
[LinAdd.org]
name=Linux Addons
baseurl=http://www.linadd.org/download
gpgcheck=0
# yum makecache

Then run the command:

yum install <package-name> <package-name> ...

Then this great tutorial is the way to do it.  Worked great for me in my practice session.