Clark's original instruction was: Eat a no-fat breakfast and lunch such as cooked cereal, fruit, fruit juice, bread and preserves or honey (no butter or milk).
I had been flushing for more than a year before I even heard about the benefits of apples or apple juice. But as a regular routine, now, my first meal is oatmeal and chopped apples, which I wash down with orange juice. (One day I had absent-mindedly drank with milk, but it didn't have a negative effect on the results). The prep I do the day before is just eating light. Later in the day it's just a potatoe with mixed vegetables.
For me, it is unnecessary. I never had a problem without it