This is meant as a quick 5 min get it going, and a 5 min quick testing to get dkim going.I do recommend actually reading man pages etc if you have extra time, but this guide should get you going.

Install EPEL repository:
64 bit:
# rpm -Uvh http://download.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-7.noarch.rpm
32 bit:
# rpm -Uvh http://download.fedoraproject.org/pub/epel/6/i386/epel-release-6-7.noarch.rpm

Install DKIM:
# yum install opendkim
# export domain=YOURDOMAIN.com
# mkdir /etc/opendkim/keys/$domain
# cd /etc/opendkim/keys/$domain
# opendkim-genkey -d $domain -s default
# chown -R opendkim:opendkim /etc/opendkim/keys/$domain
# echo "default._domainkey.$domain $domain:default:/etc/opendkim/keys/$domain/default.private" >> /etc/opendkim/KeyTable
# echo "*@$domain default._domainkey.$domain" >> /etc/opendkim/SigningTable

If you have internal hosts relaying through that you want to sign mail for to:
# echo "192.168.0.0/24" >> /etc/opendkim/TrustedHosts

Edit DNS:
# cat /etc/opendkim/keys/$domain/default.txt >> /var/named/master/YOUR_DOMAIN_DNS_ZONE_FILE
(what I normally do at this point is increment serial number in DNS zone file, login to slaves, delete their zone files and restart named there to get it going quickly)
# nano -w /etc/opendkim.conf
◦Mode sv
◦Domain YOURDOMAIN.com
◦uncomment everything except KeyFile
(Find this line: SigningTable /etc/opendkim/SigningTable and change it to:
SigningTable refile:/etc/opendkim/SigningTable to enable regex wildcards on SigningTable)

Configure Postfix
# nano -w /etc/postfix/main.cf (add following)
# opendkim setup
smtpd_milters = inet:localhost:8891
non_smtpd_milters = inet:localhost:8891
milter_default_action = accept

Restart Services
# service opendkim start
# service postfix restart
# service named reload
# chkconfig opendkim on


Test our setup
# echo "DKIM Test" | mail -s "DKIM Testing" SOMEUSER@gmail.com
# tail -100 /var/log/maillog
Now make sure maillog log shows it signed, check gmail headers of email you sent, make sure everything passes fine

—
Dan