See https://ademcirak.com/configure-server-for-sending-signed-email-dkim-spf-and-dmarc.html.

DKIM requires some knowledge about generating certificates. However, PHPMailer's DKIM examples should help tremendously: https://github.com/PHPMailer/PHPMailer/tree/master/examples

DMARC is just a record you add to your DNS after you've done SPF and DKIM. This wizard I found can help create this: https://dmarcian.com/dmarc-record-wizard/

I also found http://www.appmaildev.com/en/dkim which will help you verify DKIM, SPF, and DMARC.

Google's answer here gives some other tips to take a look at: https://support.google.com/mail/answer/81126. I haven't tried out Google's Postmaster tools, but Gmail itself does provide indication if DKIM and SPF pass.