If you haven't done a parasite cleanup in recent history, a Liver Flush isn't your best beginning. Take care of the little critters (everyone has them) prior to flushing.
I've done Liver Flushes while fasting -- if you are clean, not experiencing any kind of detox crises during your fast and feeling strong, flush away. Otherwise, wait until after the fast and do it while eating a good clean diet. It takes a lot out of you.