You have to talk to Suzune in the old town area, at two different locations, and then a third encounter in Ragdum. Once you've talked to her 3 times, an Event marker will appear at the topmost point in the Slums area (atop of a scafold near the altair on the rooftop). Access the event, and you'll be transported to the Ranch area.
While it's mostly accessed via Altairs, it is possible to then connect the Ranch to the rest of the map with a physical path: get at the bottom floor of the Ranch and keep running right, which leads to the Old Cemetary, which in turn leads to a portion of the Discarded Lower Layer.
