openssl genrsa -out dk.key 768
openssl rsa -in dk.key -out dk.pem -pubout -outform PEM
Will generate the private key (dk.key) and public key (dk.pem).
Take the public key and c&p to a txt record:
k=rsa; t=y; p=[data from dk.key file, strip BEGIN/END blocks]
The key should be named similar to
[selector]._domainkey.example.com. Note the selector as you'll have to configure your SMTP server with the same name so the receiving server knows where to look for the key (it can be anything you want, like "key1" or "asdf").
You should also configure a policy record for your domain. The record should be named
_domainkey.example.com and contain "o=-" (for all e-mail from domain must be signed) or "o=~" (for signature optional). You may also add "t=y" and some servers will be more forgiving, as it indicates you're testing.