First install OpenDKIM:

# yum install opendkim

Configure OpenDKIM (please read the comments inside the original config file):

# vim /etc/opendkim.conf

PidFile /var/run/opendkim/opendkim.pid
Mode    sv
Syslog  yes
SyslogSuccess   yes
LogWhy  yes
UserID  opendkim:opendkim
Socket  inet:8891@localhost
Umask   002
SendReports     yes
SoftwareHeader  yes
Canonicalization        relaxed/simple
Selector        default
MinimumKeyBits  1024
KeyTable        /etc/opendkim/KeyTable
SigningTable    refile:/etc/opendkim/SigningTable
ExternalIgnoreList      refile:/etc/opendkim/TrustedHosts
InternalHosts   refile:/etc/opendkim/TrustedHosts
OversignHeaders From
SignatureAlgorithm      rsa-sha256

Generate a domain key + domain cert:

# cd /etc/opendkim/keys
# opendkim-genkey -d mydomain.com
# mv default.private mydomain.private
# mv default.txt mydomain.txt
# chown opendkim:opendkim mydomain.*

Add the key to the keytable:

# vim /etc/opendkim/KeyTable
>> default._domainkey.mydomain.com mydomain.com:default:/etc/opendkim/keys/mydomain.private

Add the cert to the SignTable

# vim /etc/opendkim/SignTable
>> *@mydomain.com default._domainkey.mydomain.com

Edit postfix config and add OpenDKIM to the milters:

# vim /etc/postfix/main.cf
>> smtpd_milters = inet:localhost:8891
>> non_smtpd_milters = inet:localhost:8891

Start OpenDKIM:

# systemctl start opendkim
# systemctl enable opendkim

Restart postfix:

# systemctl restart postfix

Add DKIM TXT record to DNS (Bind9):

# vim /var/named/mydomain.com
>> default._domainkey.mydomain.com. 3600      TXT        "v=DKIM1; k=rsa; p=<very long string taken from /etc/opendkim/keys/mydomain.txt>"

Restart Bind:

# systemctl restart named

Well, that should be it, wokring DKIM on your mydomain.com!