The one thing I would say is that it's a good idea to install the Mail::Audit module first. So;
perl -MCPAN -e shell
(answered 'no' to the 'do you want to muck around with the CPAN defaults' question; I'm no great shakes with Perl.)
install Mail::Audit quit
(ftp.perl.org was having a funny five minutes when I installed; I hit ctrl-c and it found another server (in Demon) and got Mail::Audit off there instead.)
I also made a point of installing Net::DNS, just for the hell of it; identical procedure to the one above.
You can then either download spamassassin as a tar.gz from http://spamassassin.taint.org/downloads.html or type;
perl -MCPAN -e shell install Mail::SpamAssassin quit
if you do this, it autoinstalls
If you go the tarball way, then tar -zxvf it; perl Makefile.PL; make; make test; make install
(which shoves it in /usr/lib/perl5/site_perl/5.6.1/ on my box.)
Tried doing the tests in READMEand all worked fine from the off.