I've blogged about using OpenDKIM with Postfix, having myself run the two together for a quite a number of years now. I've set up enough servers from scratch to have compiled my own setup guide. You'd find it cryptic and confusing, so I won't share it. But what I can share instead is this wonderful guide from EasyDMARC that explains how to configure OpenDKIM to add DKIM support to your Postfix install. I'm so glad they put this all together, and if you're the right type of email nerd, or want to become one, this is something that you will want to bookmark. Read it here.