First you should have the 'SSH' login permission to the server to start/stop the CSF. Login to the server through ssh using a Terminal or Putty. 

To start the CSF, run the below mentioned command. 

/etc/init.d/csf start 

To stop the CSF (if it is already functioning), then use the below placed command. 

/etc/init.d/csf stop 

To restart the CSF, run the below command. 

/etc/init.d/csf restart