If you are doing a complete coolant change, which you should because of age if original or because it is NOT a good idea to mix different coolant chemistries, you really should buy the concentrate, not 50/50. The reason is that you can never (reasonably anyway) get the several gallons of residual water (hopefully distilled water for final rinse) out of those long hoses from engine to dash heater and the heater core itself.
If you want "blow by blow" instructions on doing it properly, let us know. It is NOT difficult to do properly but is time consuming.