Hi Monica -
I have not done this one myself but did some digging out of curiosity. See if this is helpful at all?
This website....
https://civitavecchia.portmobility.it/en/taurine-baths-civitavecchia
....provided instructions on how to visit the site yourself using the local bus, and the web address and email of a local, non-profit tourism resource. The site is only in Italian so use your online translation tool.
https://www.prolococivitavecchia.com
[email protected]
You might send them an email and see if they can suggest a local guide and/or further instructions for a DIY tour starting from the Civitavecchia rail station? I guess I'd want some sort of assurance that the site will be open on the day you wish to visit, and unless the portmobility website is in error, it looks like the hours the site is open are limited.
I might guess that taxi from the train station is also an option - the site isn't that far out of town - but I've no idea what it might cost to have them wait.