How to generate DKIM Keys

How do I generate DKIM keys for a third party email service. Most big providers has tutorials, but I’m not finding anything for a small email provider. I’ve seen that puttygen can generate SSH keys. I believe the domain name should be… frii._domainkey.co.washington.co.us as mail.frii.com is our dns provider (and former email provider).

Thanks for any guidance and/or assistance.