How often should you add antifreeze

Depends on several factors. If you have plain water in your radiator/cooling system and you dont know how much it holds you are going to be hard pressed to get the right amount of antifreeze in it. You need to drain some of the water off and add coolant straight to the radiator. I would drain it all and put in new pre-mixed coolant. Dont ever put tap water in your cooling system.

Break free 2 bolts on thermostat housing, loosen hose going into housing and clamp, it will leak some antifreeze. Once the housing is taken off block clean off old gasket material from both surfaces. The block and housing, be careful not to scratch the surfaces. No sharp pointy objects, aluminum is soft. Then take the old thermostat out and put in the new one. Put gasket in place ,align housing and put , bolts in by hand first, so it doesn't cross thread. Just snug bolts and top off system with antifreeze . Start engine ,put on heat and run until thermostat opens. Check for leaks, tighten bolts 1/4 turn should be fine. Add antifreeze in overflow bottle if needed.

As far as I know there are no drains in the cooling systems any more. I took the bottom rad hose off and drained my Buick. They say you should not put water in car rads any more.

Add anitfreeze in the top of the radiator. Lift the hood in front, take off the radiator cap and add antifreeze until it is full. Purchase Prestone
50/50 pre dilluted antifreeze solution at WalMart.

My toyota the same way. I add antifreeze to radiator it work know

It will need as much as it takes, which is impossible to tell depending on how you flushed it.
But your whole system takes more than 2 gallons, and since you want a 50/50 mix you know you will need more than one gallon of antifreeze. So you need to start by purchasing 2 gallons. If you have left over, it does not go bad. And if you do not quite have enough, you can probably add a little extra water because you probably did not flush all the antifreeze out.
Once you run it, then next time it will need more again after the heat bleeds the air out.

Yes, water works fine. You just will need to add some antifreeze to the system before freezing weather arrives. If you have antifreeze in the vehicle and are just diluting it some, then drain it about halfway and refill with antifreeze when winter arrives. If it is empty and you are just putting water only, then if your engine is an alluminum block and head, it will be fine for now with just water. If you have a cast Iron block, then you will need to add antifreeze to lubricate and prevent your block from rusting on the inside!

Put antifreeze in the container. Start the car. When fluid level goes down, kill it, put more antifreeze in. Repeat until filled.

Most vehicle Mfg.'s do not want you to access the radiator, so they put in overflow tanks.
You can add coolant there. You will find it under the hood next to the interior fender.
Only check or add coolant when the engine is cool.
I would start looking for a coolant leak. Vehicles are not supposed to leak or lose their coolant.
Check for green coolant spots on the ground where you park your vehicle.
Please let me know what you find.