How Do I Create A Custom Lot Type?

I'm wanting to make a new lot type (there are Library, Bar, Park, etc. options). I want to make a Photography Studio, but no lot type seems right, so I'd like to create a new lot type. I know it's possible, since Zerbu's Go To School mod added High School & Elementary School lot types. It seems like it would be easy enough - just set the lot type category name, set visitor restrictions, and build requirements - but I don't know what tool to use. Please help!