Place all ingredients together in a large pot.
Bring to a boil and then simmer with the lid off until the water level is reduced by a few inches (expect this to take 1 ½ – 2 hours)
Strain the pot contents through a sieve into a measuring cup or bowl with a spout.
Pour strained liquid into ice cube trays to freeze, or into sterilized jars/bottles for refrigeration (concentrate will last for 2 weeks in the fridge)