The basic formula is pretty simple: Blend about 12 ounces of liquid with two "glassfuls" (see recipe below for the photo) of ice cubes to the count of 30.
For a specific example, here is how to make a root beer slushie, similar to a 7-Eleven Slurpee or a fast food Icee. I'll provide some visual aids, err, photos:
1. One can of soda, usually 12 ounces, is good enough for two slushie servings. Pour as much as you want into fridge-friendly containers, and let the soda flatten for a few hours. You could always substitute this recipe with about 12 ounces of any blend of liquids, fluids, and beverages.
2. You'll need two "glassfuls" of ice to blend two slushie servings. I use a plastic tumbler that's about the same size as a travel coffee mug. I only use refrigerator machine "cubed" ice, and I have been satisfied with the results. I don't know if ice tray cubes will have similar results. I do know that crushed ice won't do, however.
3. Pour 12 ounces of flat root beer with two "glassfuls" of ice cubes into the NutriBullet container. Sprinkle some sugar to sweeten the ice; don't go overboard, though. You just want the sweetness level to remain on par with the original drink, if you're using flat soda.
4. Close the NutriBullet container with the six-blade cover and blend to the count of 30. If the mix is correct, the container should be packed to the blade but loose enough to have space to mix and blend. A good blend will continuously blend for the entire count of 30. A bad blend will have the blades blend nothing for some part of the count of 30. These proportions are not recommended by the manufacturer, so your results may vary. In other words, don't blame me for any NutriBullet-related accidents!
5. Scoop or pour carefully into two relatively large glasses (over 12 ounces), make more if needed, and enjoy!
The end. Cheers!