If you buy pre-mixed coolant you do not need to add water. If you buy straight coolant than you should mix it with water to make 1:1 solution.
Depending on the make and model, fill to the proper level in the coolant reservoir. Then pressure check, bleed system and re-check to make sure it's at the proper operating level. For a 2004 Chevy Monte Carlo the total volume of 50/50 coolant and water will be more than 11qts.
The owners manual says the total capacity of the system is 11.7quarts. I usually use an empty 1 gallon milk jug. fill it halfway with water and the rest coolant. And keep doing that till the system it topped off. That way there is no guessing at how much was left in the system vs how much water and coolant to add.
One thing you might want to consider is that there will be an air pocket in the system. If you just add coolant and drive you probably won't have heat and you run the risk of over heating the engine. You will be able to fill the system with the car off and the level will appear to be full. But to ensure that the air pocket is removed you will need to run the vehicle with the heat on and radiator cap off until the thermostat opens. I usually just wait until the cooling fans come on, top off the coolant (while the vehicle is still running), and make sure I have good heat at idle. Don't worry about measuring the amount of coolant you put in. When the system is bled and full it will over flow and the Over flow bottle is marked. Fill that to the hot line, wash up anything that spilled, cap the system and enjoy your car. Best of luck!