the scripting is actually the easiest part of it all, since there isn’t much actually. all you need is to copy an existing industry and change its name and the name of the consumed/produced goods, then add these goods to the list of goods and add them in the wagons that should transport these.
since there are some hardcoded limitations to the supply and demand system, i would generally suggest to keep things as simple as possible. so as goods you should use something like “livestock” or “grain” instead of say “cattle” or “wheat” and the final product always needs to be “goods” because thats the only thing consumed by citys and if you would specify any other product you couldn’t create any demand for it at all.