Adding a DKIM record to your DNS is quite simple. Add two txt records like

_domainkey.domain.com          IN TXT o=!;r=postmaster@domain.com
selector._domainkey.domain.com IN TXT v=DKIM1;k=rsa;p=<public key>

Which means:

• o=~ the server signs some mail
• o=- all mail is signed, but unsigned mail should be accepted
• o=! all mail is signed, do not accept unsigned mail
• t=y I’m still testing
• v=DKIM1 we use DKIM version 1
• k=rsa it is a RSA key
• r=<x@xx> report problems to this email address
• p=<public key> this is my public key