If your car has a radiator with a radiator cap as well as the coolant reservoir then you open the radiator cap and fill the radiator up with the coolant.
Then also add about half coolant to the coolant reservoir.
If your vehicle is just equipped with a coolant reservoir that leads into the radiator and you don't see a radiator cap then you add the coolant to the coolant reservoir.
If you do need to open the radiator cap make sure the radiator and coolant has cooled off enough.
I like to wait at least an hour to allow the coolant and engine to cool off and for the pressure to be released from the coolant reservoir or radiator before opening up the radiator cap.
You can also buy radiator caps with a handle that releases the pressure.