Wash the grapes and deseed them by cutting each grape in half and carving out the seeds. Alternatively, you can add the grapes with 300 ml of water to your blender and blend for 1 minute, then strain the liquid through a fine sieve or cheesecloth. I do prefer the first method, though, because more of the grapes' flesh and skins end up in the smoothie (as opposed to ending up in the sieve or cheesecloth).
Add the deseeded grapes and the remaining ingredients to your blender. Blend on high speed until smooth.