In a mason jar, combine the filtered water, honey and salt. Use the handle of a wooden spoon to give this a good stir.
Add the berries to the jar and place a fermentation weight over the berries to submerge them below the brine.
Loosely place the lid on top of the jar and place the jar in a cool, dry place for 24-48 hours (see note).
When your berries are ready, place them in the fridge where they should be good for 2-3 months.