If you need to get CVS, start here.
If you need to learn about CVS, read this great reference manual.
To checkout the current source tree from the CVS server:
% setenv CVSROOT ':pserver:firstname.lastname@example.org:/squid' % cvs loginWhen prompted for a password, enter 'anoncvs'.% cvs checkout squid3
You can use cvs -d if you are afraid of csh or don't like to set environment variables.
To access older Squid releases use the same procedure as above to login and then checkout the specific version sources