How to set Cassandra volumes?
commitlog_directory and data_file_directories (use this link if you are on 3.0+) will tell Cassandra where to look for commitlog and data files (there are a few other directories you might be interested in, search through the documentation page for 'director'). However, your volumes have to be mounted first to be able to put any directories onto them. 

How to mount them is not really a DB question, though.  It is described in any case in the EBS documentation.