As this is the identification stage at this point it is only necessary to isolate the cold tap. Most of the time that is achieved by turning off your 'stop tap'.

Turn your cold tap on so water is gently running and then turn off the incoming main (stop tap) so the water stops running. If the tap you're fixing is downstairs, go upstairs and open a cold tap. This allows any water in the system to drain down and stops if flooding out when you remove the cartridge.