Put 400ml cold water into a large jug or jar and add the coffee grounds, but do not stir the mixture. Cover and leave to steep for 24 hrs.
Gently stir the mixture, then pour it through a funnel or sieve lined with coffee filter paper (or a sheet of kitchen paper) into another large jug. This concentrate will keep in the fridge for 2-3 days.
To serve dilute 1 part cold brew with 2 parts still or sparkling water (or to your taste) and serve with lots of ice.