I help organize a workshop in a conference on machine learning where the conference doesn't publish proceedings for the workshops: the workshops have to do it themselves if they want to.

What are the steps to find a publisher and strike a deal to publish the proceedings of a workshop? (where to find the list of publisher in a given field, what are the typical contract clauses and negotiation points, etc.)

I know that we can publish it ourselves (since in the end publish = hosting some PDFs online) but I'm curious to know how the old-fashioned publishers work.

If that matters for the answer, all papers are written in English and one requirement is that papers are open access or allowed to be posted on arXiv.