Start with the milder vegetables, cucumber and celery. Work up to the spinach, then add bitter greens, like kale. Finally, squeeze in the lemon (or lime) to cut the bitter flavor.
Place ingredients in the juicer.
I recommend drinking your juice right away (or morning of) ... they do not last more than 24-hours! It has also been shown that the best time to drink green juice (especially celery juice) is on an empty stomach and first thing in the morning.