I've have this plant for a month or two and I'm not sure if it's under or over watered. I've only watered it once, but it's squishy to the touch, and if you look closely, you can see sagging horizontal lines on it. I'm new to this species so I don't want to kill it by watering it!

Dont think so. It ought to be much flatter if its actually thirsty. To quote "when in doubt , do not water"