2000 Mercury Sable tune up problem

I recently changed the plugs on my car because I was told that was why my car had a loss of power particulaly when I would go up a hill. Anyway changed plugs I was carefull not to cross any wires and did one plug at a time. Since then my car seems to run roughly and when I sit still at a light or something it feels like the motor jumpsl. What could I have done wrong.