Without knowing what is currently in there it's best to just add distilled water to bring it back up to the full mark. When you can, flush it and refill it with a 50-50 mix of distilled (only!) water and something like Zerex G-05 which is a superb HOAT coolant.
He's in Michigan.
Toximus - You do the Prestone first? Then the Thermocure for how long? ONce you drain out the Thermocure the you fill it back up with 50/50 Distilled Water and G05 coolant?
To be perfectly honest, I don't know what flushes work and what doesn't. So I try to cover my bases. This is my theory: The Prestone is used primarily to remove oils, dirt and grease that may have found it's way into the system. The Thermocure is used to remove rust and mineral build up in the system. You'll need 1 bottle of each.
Always collect and properly dispose of the coolant, chemical cleaners, and the junk that comes out of your radiator.
First fully drain the system and flush it once with just distilled water to remove coolant from the half of the system that didn't drain (it costs more, but I don't use hose water even for flushes). You don't want coolant in with the chemical cleaners. Every time you start the engine for the following steps you need to allow the coolant to burp — a funnel helps so it doesn't go everywhere.
I then do the Prestone following the instructions on the bottle (run for 3 days and get it up to temp several times or something like that) and fully flush with distilled water after to fully remove the chemical.
Then in goes the Thermocure following the instructions (run engine for an hour trip or something like that) and flush with distilled water until the water comes out mostly clear (it never comes out as good as it went in). If you need more distilled water go on trips to the store to buy more with just the distilled water in the system. If it doesn't start clearing up after 20 gallons you probably have a lot of build up or somebody used leak stop in the system before and it's breaking up. If you need to, use another bottle of chemical flush, replace parts, or whatever is necessary to fix it — I had one that needed a heater core after running chemical flush because the PO used leak stop.
Once you have just distilled water in the system drain the radiator and add the coolant concentrate. Dump the overflow bottle and pour in the mix (look at the chart on the bottle, you may want to do a 60/40 mix or something else). Let it all mix up by running the engine for a while and then use a coolant tester to test if the mix is correct (I don't have one that I recommend. I normally buy 2 different styles cheaply from the auto parts store in case one isn't accurate). Only test the mix through the radiator cap — the mix in the overflow may not be the same. You can compare the mix in the engine to the mix you made in the bottle to know if it's correct. Add or subtract coolant/water until you get the mix where you want it. Lastly, check the mix again in the overflow.
It's a pretty simple job after you've done it once, but it's always messy so plan it for a warm day when you can be outside.
