Roughly chop the cucumber and celery, quarter the apple, and half the lemon.
Add all of the ingredients to a blender, start blending on low, slowly turning up the speed.
Blend on high for 60 seconds, until you have a very smooth juice.
If you are using a nut milk bag, put the bag inside a pitcher and pour in the juice. Squeeze the bag to extract the juice, only being left with the pulp.
Store in the fridge for up to 3-4 days.