Add the kale first to the blender, then the frozen banana, and then the remaining ingredients.
Blend everything for about 2 min (or longer if you think it is not yet smooth enough. But I do recommend you leave the blender resting for about 1 min before you turn it on again).
Recipe Notes
* I used frozen bananas (that I had previously frozen). The smoothie always gets warm in the blending process and to keep it cold, and thus preserve more vitamins, I like to include a frozen fruit.