From Setting up SPF, SenderID and DKIM on Centos 5.3 using sendmail (2009) and Installing OpenDKIM RPM via Yum with Postfix or Sendmail (for RHEL / CentOS / Fedora) (2011)

Step 0. Install opendkim

yum install opendkim

Step 1. Generate a private key

As root: 
mkdir /etc/opendkim/keys/terraswarm.org
/usr/sbin/opendkim-genkey -D /etc/opendkim/keys/terraswarm.org/ -d terraswarm.org -s default
chown -R root:opendkim /etc/opendkim/keys/terraswarm.org
chmod 640 /etc/opendkim/keys/terraswarm.org/default.private
chmod 644 /etc/opendkim/keys/terraswarm.org/default.txt
FIXME: The edits are too complex, see Installing OpenDKIM RPM via Yum with Postfix or Sendmail (for RHEL / CentOS / Fedora) (2011)

Update /home/www/php/etc/sendmail-moog.mc

Edit /home/www/php/etc/sendmail-moog.mc: 
INPUT_MAIL_FILTER(`opendkim', `S=inet:8891@127.0.0.1')

cd /tmp/mail
cp ~www/php/etc/sendmail-moog.mc sendmail.mc
m4 sendmail.mc > /tmp/sendmail.cf
diff sendmail.cf /tmp/sendmail.cf
Typically, sendmail.cf has been edited by hand. Update ~www/php/etc/sendmail-moog.mc, copy and rerun m4 until the diffs look ok. Then do: 
cp /tmp/sendmail.cf /etc/mail/

Start it up

[root@moog mail]# service opendkim start
Generating default DKIM keys: [  OK  ]
Default DKIM keys for eecs.berkeley.edu created in /etc/opendkim/keys.
Starting OpenDKIM Milter: [  OK  ]
[root@moog mail]#
Restart sendmail 
service sendmail restart
"If everything looks good, I recommend running chkconfig on OpenDKIM to make sure it starts when you boot your server:" 
chkconfig opendkim on

Update the dns records

[root@moog mail]# cat /etc/opendkim/keys/terraswarm.org/default.txt
default._domainkey      IN      TXT     ( "v=DKIM1; k=rsa; "
          "p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDwAClvdkznCFP3mOdsgIH+iWXo/hpRwQbiGatgoTO7Bz0Jca4fwL6gdkwsQ7eCG7MNwsxcnfPHciJL9AAbaR+79bP+nv8RsTThOEQi+Gk24lInAz1L1S6TNdBFXkvHSuQi2eRY6AEb8LPPZ6rAgwaTDHPUn30yh3LWaoYVrWIwIDAQAB" )  ; ----- DKIM key default for example.com
[root@moog mail]#

Step 3. Create a DNS record of type TXT

Terraswarm's DNS is served from http://ehosts.com.

The account name is EdwardAshfordLee

I had to place a support request to get this updated. To check, use dig. Note that dig any terraswarm.org does not show you the record! 
[root@moog 10]# dig default._domainkey.terraswarm.org TXT

; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.30.rc1.el6_6.1 <<>> default._domainkey.terraswarm.org TXT
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 39519
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 2

;; QUESTION SECTION:
;default._domainkey.terraswarm.org. IN  TXT

;; ANSWER SECTION:
default._domainkey.terraswarm.org. 3600 IN TXT  "v=DKIM1\; k=rsa\; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDwAClvdkznCFP3mOdsgIH+iWXo/hpRwQbiGatgoTO7Bz0Jca4fwL6gdkws/Q7eCG7MNwsxcnfPHciJL9AAbaR+79bP+nv/8RsTThOEQi+Gk24lInAz1L1S6TNdBFXkvHSuQi2eRY6AEb8LPPZ6rAgwaTDHPUn30yh3LWaoYVrWIwIDAQAB"

;; AUTHORITY SECTION:
terraswarm.org.         901     IN      NS      ns1.ehost.com.
terraswarm.org.         901     IN      NS      ns2.ehost.com.

;; ADDITIONAL SECTION:
ns1.ehost.com.          3432    IN      A       65.254.254.120
ns2.ehost.com.          78510   IN      A       65.254.254.121

;; Query time: 85 msec
;; SERVER: 128.32.48.21#53(128.32.48.21)
;; WHEN: Tue Mar 24 17:00:34 2015
;; MSG SIZE  rcvd: 375

This DNS record will be retrieved by mail receivers who want to verify emails with DKIM signatures. The record name default._domainkey tells verifier that the selector of this signature is default, therefore if you are changing selector name to something else, make sure you change all of them consistently. FIXME: Not sure what to do here