Juicer method: run the fruit through a juicer, then stir in the lemon juice.
Blender method: blend the fruit with the water, then strain through a fine sieve or nut milk bag.
Taste and adjust with more grapes or a little honey if too tart.
Pour over ice and serve immediately.
For a blackcurrant cordial, simmer the blackcurrants with the water and a little extra sweetener until they burst, strain out the skins and seeds, then bottle the syrup and dilute to taste with still or sparkling water.
Notes
Use seedless grapes or strain out seeds. Frozen blackcurrants work well. Best served fresh; use maple syrup to keep it vegan.