Grapes halved, seeded and simmered in water with sugar, then cooled and strained. It is a cooked juice rather than a blended one, which is what makes it keep and what concentrates the flavour.
This is a cooked juice, not a blended one, and that is the difference between it and most of the juices on this site. Twenty minutes of simmering breaks the grapes down, pulls the colour out of the skins and concentrates the flavour in a way a blender cannot. It also means it keeps for several days in the fridge rather than separating within the hour. Let it cool completely before straining. Hot juice poured over ice melts it immediately and you end up with something watery, and straining while hot pushes pulp through the sieve and clouds it. Seeded grapes matter here. Grape seeds are bitter and twenty minutes of simmering pulls that bitterness straight into the liquid, which is why the first step takes them out rather than leaving it to the strainer.