Download and untar source files

Install lame mp3 encoder (not recommended) if you plan to make use of mp3's for MOH.  This is now a legacy feature as the default behaviour of Asterisk/Freepbx is to NOT use mp3's for anything.  Freepbx by default now automatically converts uploaded mp3's to wav.
rpm -ivh http://apt.sw.be/redhat/el5/en/i386/RPMS.dag/lame-3.98.2-1.el5.rf.i386.rpm

Install FreePBX.  Check if this is the latest released version.
cd /usr/src
wget http://mirror.freepbx.org/freepbx-2.8.0.tar.gz
tar zxvf freepbx-2.8.0.tar.gz

Install Asterisk and addons.  Check if this is the latest released version of Asterisk and Asterisk addons
wget http://downloads.asterisk.org/pub/telephony/asterisk/releases/asterisk-1.6.2.13.tar.gz
tar zxvf asterisk-1.6.2.13.tar.gz

wget http://downloads.asterisk.org/pub/telephony/asterisk/releases/asterisk-addons-1.6.2.2.tar.gz
tar zxvf asterisk-addons-1.6.2.2.tar.gz

wget http://downloads.asterisk.org/pub/telephony/dahdi-linux-complete/releases/dahdi-linux-complete-2.4.0+2.4.0.tar.gz
tar zxvf dahdi-linux-complete-2.4.0+2.4.0.tar.gz

wget

http://downloads.digium.com/pub/libpri/libpri-1.4.11.4.tar.gz

tar zxvf libpri-1.4.11.4.tar.gz