First, rinse the currants in water and drain well.
Sterilize your jars by first washing them with soap, then leaving them in the oven at 100C/212F for 15 minutes. Take them out of the oven right before you are ready to fill the jars.
Put the currants in a large pan with the water. Put the pan over low heat, then gently bring to a simmer. This may take some time, even an hour. Do not stir. When it boils, turn off the heat and leave to infuse with the lid on. They are finished when the currants have changed colour (greyish red).
Strain the currants through a muslin without pushing the berries. This will take about 10 minutes. If you squeeze with you hands, the cordial will turn cloudy.
Measure the amount of liquid to find out how much sugar you need. You should add at least ⅓ part sugar to 1 liter berry liquid. This will make a sweet-sour cordial.
Return the liquid to the pan, this time with the sugar. Boil briefly, until the sugar is dissolved, about a minute.
Skim off the foam and pour the cordial on sterilized jars. I always squeeze the muslin with my hands to get the last remains of the juice, and then bring to boil with some sugar. This cloudy cordial I then keep in a little jar in a fridge to consume first.
