Combine all ingredients in a blender and blend at a high speed until slushy and large chunks of ice/frozen juice are chopped. Don't over blend or it will be really liquidy.
Pour desired amount of juice in an ice cream maker and churn until thick and icy. In my Cuisinart Ice Cream maker it takes 10 minutes.
For all methods you can swap out part of the juice with flavored sparkling water for a little more of a bubbly texture (this will make it less sweet). Use any combination of juice and/or frozen fruit you have on hand and enjoy! My boys had lots of fun personalizing their flavors.
We found the Juice Blender Method to be the most slushie-like, which was my boy's favorite. The Frozen Fruit + Juice Method was still enjoyed but has more of a smoothie texture.