To make use of these, a coupler is simply placed into each icing bag. Normally you would require quite a few nozzles for the various colours you’re using and you would have to finish all your piping in one colour, then remove the nozzle and wash it before proceeding with the next colour. A coupler allows you to change only the nozzle (e.g. if you’d like to do some outlining with black icing using a round nozzle, then pipe stars using the star nozzle, you can simply change the nozzle by unscrewing the top of the coupler; normally you would need two separate icing bags). The best way to explain how to insert a coupler is via pictures.
Now you are ready to load the icing bag with your icing. Place each of the icing bags into a jug or glass to make it easier to load the icing into the bag. Twist the icing bag just above the top of the icing and use an elastic band or a clip to prevent the contents from oozing out or the icing from drying.