How to change water pump?

i have a 1993 toyota camry. it keeps over heating i changed the thermostat and tested the old and it is good sometimes when the engine is cold and you put it in gear it makes a little puddle and drops for about 30 seconds it doesnt over heat if it is at iddle only when you are driveing i know the heater core is good so the water pump makes sence becuase when the engine is warm the water would evaporate before i could see it. also how do i change it.