I don't know the colonix product, but I've prepared and flushed my liver several times. The basic requirement is to have rapid movements through the digestive tract. If you eat something, it should come out "the other end" within 24 hours. One way to measure this is to eat something that will pass through relatively unchanged and wait for it to come out. Whole flax seeds works, if you swirl it in water and drink without chewing.
Basically, you want your elimination system working well enough to rapidly eliminate everything you flush from your liver, so it isn't reabsorbed while in the small intestine.
Adding enzymes, bulk and soluble fiber to my diet helps move things along. The "whole foods" way to do that is eat a fresh, whole apple before every meal and one before bed. The supplement alternative is to take enzymes, psyllium and pectin 4x per day.
The Epsom Salts definitely wash through your system, so make sure to take all four doses. When people skip the last dose, they seem to get sick afterward.
The Liver Flush isn't nearly as fearsome as it seems -- for strong and reasonably healthy people.