You can flush without preparation.
Preparation helps move your stagnant bile and therefore your flush is likely to work better, but it is NOT a must.
You should benefit from it, yes! No harm att all, in my experience.
I started flushing without using any Epsom salt, later I started using a tablespoon of it and I could notice it helps dilate the bile ducts. However, Mrs Chang from http://www.sensiblehalth.com recommends no E.S. as she says is hard on the kidneys. Read her website, very educational and I am sure you´ll benefit from it.
Important: Watch what you eat the day after the flush: Only well cooked veggies and lots of sea-salted water.