Remove any tough stems from the spinach and trim ends off the celery and roughly chop.
Add the apple juice and coconut water/water first, then add the spinach, pineapple and celery to a high powered blender - make sure your blender is OK to blend ice/frozen fruit. Add yogurt, if using.
Blend until smooth then serve, enjoy immediately while still chilled.
Notes
If you have fresh rather than frozen pineapple, I'd recommend using ice rather than water to help chill the drink.