What are the steps to find a publisher and strike a deal to publish the proceedings of a workshop?
I went through this a few years ago for a conference for which I was the program chair.
Since you want open access, you'll find that the big commercial publishers will want to charge quite a bit (thousands of dollars per paper) to publish your conference proceedings.  For open access publishing, there are a few reputable publishers and lots of disreputable publishers that you'll want to avoid.
One reputable publisher for topics likes yours is Dagstuhl's LIPICS series.
If the society that sponsors the conference has its own in-house publishing arm then that can be a good choice too.