cd /usr/src && wget http://www.configserver.com/free/csf.tgz && tar -xzf csf.tgz && cd csf && sh install.sh

test whether you have the required iptables modules:
perl /etc/csf/csftest.pl

Once this is done, login to WHM, take out of testing mode and set server settings as you please.

Have fun!

-Radar