Basically it's a couple of cups of Epsom Salts and a cup of baking soda in a hot bath. You soak in it for 20 minutes to an hour.
You can add things to make you sweat like ginger and taking niacin. You can add ACV or H2O2. But for me it's just the es and soda. I used to use it a lot when I had my Amalgams replaced, to detox the mercury. Now I just use it once in a while when I'm feeling depressed because it helps me sleep great and the next morning I'm feeling on top of the world. Or I'll do it if I feel chilly at night.
I learned about it on the bath forum. I guess there are as many variations as there are people who like to experiment.