procmail and new grammar in smtpd.conf

[Eda Sky]

Hi
I'm preparing an update from 6.3 to 6.4 and fix the required configuration 
files
For many years I've been using fetchmail/procmail and I do not know how to 
overwrite smtpd.conf to a new grammar
the original rule is
accept from any for domain "example.com" alias <aliases> deliver to mda 
"/usr/local/bin/procmail -f -"

I do not know how to write new rules.
Everything I'm trying to do ends with syntax error.

Will anyone advise me?
Thank you