I use OAT (long life organic acid technology)
https://www.halfords.com/motoring/engine-oils-fluids/antifreeze/halfords-oat-ready-mixed-antifreeze-5-litres.
Get the ready mixed as it takes guesswork out of mixing, 50 % is the design mix, it is bit counter-intuitive but if antifreeze is too dilute or too concentrated it will raise (not lower) the freezing point.
I have changed coolant on many cars, but I avoid unscrewing plugs from cylinder blocks as too much to go wrong and they are normally a PITA to get to. The plug could leak after it has been taken out and put back, what are you gonna do it you strip the thread when taking it out or crossthread it when putting it back in ? Could get expensive.
I just turn heater to cold (to block it off from main system) and drain out what wants to come out through the tap in the bottom of radiator ( no need to remove undertray) - I just drain into a tray and measure how much comes out (get some cheap plastic mearurring jugs from pound shop or similar)- Measuring what comes out means you know how much should go back in, if you cannot get the same amount back in you probably have an airlock in system. Put the rad cap back on and run the engine and then try to get the rest back in. It helps if front of car is higher than the rear - I normally face our cars up our sloping driveway.
The reason for turning the heater to min temperature is to keep the old coolant in the heater in the heater radiator and stop air getting in (MK2 onwards with 'fly by wire climate control probably shut the electric powered valves / solenoids when ignition power is off anyway).
When you are happy that the same amount went in that came out run the engine and turn heater on to circulate coolant properly. You probably change about half the coolant by this method so I normally run the car for a few days or so and try to cover a few miles and then repeat the procedure.