Put a bunch of grapes in a freezer-safe bag for 12-24 hours. You want them to be completely solid.
Then grab your blender and add the grapes a handful at a time. Many blenders can only handle a little bit of frozen slushy in the bottom before the blades stop working, so clean it out as necessary.
You may also find that a little bit of almond milk makes for a smoother texture and keeps it from clumping.
Blend and enjoy!