CDR-Stats comes with a script install-all-cdr-stats-freeswitch.sh which will install FreeSWITCH with CDR-Stats. In the scenario where you have already FreeSWITCH installed, you can execute the install-cdr-stats.sh, it will prompt you with a question to know if you want to run an installation for FreeSWITCH or Asterisk.
cd /usr/src/ ; wget --no-check-certificate https://raw.github.com/Star2Billing/cdr-stats/master/install/install-cdr-stats.sh -O install-cdr-stats.sh ; bash install-cdr-stats.sh
I will also advice you to read carefully the install-freeswitch script, cause it does few things apart installing FreeSWITCH for instance configuring it to be used with mongodb_cdr.
https://github.com/Star2Billing/cdr-stats/blob/master/install/install-freeswitch.sh#L166