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.