Making hard cheese from milk by vinegar
Hard cheese usually uses rennet, and a mechanical cheese press to extract as much whey as possible, and usually a long aging (drying) time to make it hard.

I think there may be a discrepancy between the original poster's definition of "hard" cheese and what is generally known as hard cheese like Grana Padano, Cheddar, and others. 

The process the OP is describing sounds like how one would make fresh paneer, and "hard" in this case might be more like the texture of "firm" tofu.