How do I use SpamAssassin with procmail?
See the procmailrc.example file in the distribution:
http://spamassassin.org/dist/procmailrc.example
spamc can be used instead of spamassassin, if you intend to install and use the spamd server system-wide.
Read 'man procmailrc' and 'man procmailex' for futher details and explanation.