What I've done in the past is, first drain radiator and if possible engine block drain plug, remove the thermostat and reattach housing, (you're putting in a new one anyway), place a coolant T junction that accepts a garden hose fitting to a heater hose (return line if I recall); add garden hose and flush. Then stop water, you can add a bottle of CLR (Calcium, Lime, Rust dissolver) at this point to the radiator, button everything up and run motor until CLR is very well distributed; let sit for a couple hours then flush with garden hose again.
Drain as much of the plain water as you can with compressed air if possible, add new thermostat and refill with correct coolant amount concentrate and then the remainder with distilled water.
If you try and add 50/50 coolant mix, you will find your coolant is less than 50/50 because of the plain water left in the system.
https://www.amazon.ca/Prestone-AF-KI.../dp/B000CCFY5W
Cheers
Sidekick Tony