Using a blender or smoothie maker, blitz the frozen berries, stevia and water with the blade until all the berries have been pureed.Adjust sweetness to your taste.Add the chia seeds and pulse a couple of times to break some of the chia seeds up (this helps them swell) or if you like whole chia seeds in your drinks, simply add them then shake or stir.Allow the sugar free berry chia fresca to sit in the fridge for 10-20 minutes to allow the chia seeds to absorb the water and swell.Shake once or twice to distribute the swollen chia seeds.