Prepare your frozen fruit: If you don't have pre-frozen fruit, chop fresh fruit into smaller pieces and freeze them on a baking sheet for at least 4 hours, or until solid.
Add ingredients to your Ninja blender: Start by pouring your chosen liquid into the blender jar. Then, add the frozen fruit, any optional additions like yogurt or spinach, and your sweetener if using.
Add ice and blend: Top with the ice cubes. Secure the lid on your Ninja blender and start blending on a low speed, gradually increasing to high. Use the pulse function if needed to break up larger chunks.
Achieve desired consistency: Blend until smooth and creamy. If the mixture is too thick, add a splash more liquid. If it's too thin, add a few more frozen fruit pieces or ice cubes and blend again.
Serve immediately: Pour your perfectly blended slushie into glasses and enjoy right away for the best texture and flavor.