How do I turn a cabinet into a farmhouse sink?

30" x 19 and 9" deep

I think it would be worth a trip to Home Depot with your measurements. The comments by people who've brought this cabinet say it can hold a couple hundred pounds, but if your sink is cast iron... they can be 300 lbs.

There are lot videos on YouTube on how to install, especially by manufacturer. And your manufacturer's website would probably have either a video or instructions how to install on it. Generally some sinks require 4 upright supports screwed into your cabinet, some require 3 horizonal supports screwed in, and the heaviest require supports directly underneath the sink to hold it up, sometimes 2 metal bars. The newer cabinets with a front drawer or drawer front usually can be removed for the sink apron to fit in but measure that depth.

If you want something more country, I would find a local architectural recycler. They pull out everything from buildings including old wood cabinets that you can adapt and some of them are beautifully milk painted that holds up for decades. Habitat for Humanity Restore also has used wood cabinets.