Fill a bathtub half full of warm water. Pour the ingredients into the tub, then add more water, swirling it around with your hand to spread out the bubbles. Now indulge and enjoy.
Please know that almost all inexpensive as well as the more expensive brands of shampoo are highly detergent because they are made to clean hair which is more oily than other body parts.
Most detergents are too strong for the more delicate body parts, and are likely to cause irritation at the very least. Tender muscous membranes are not meant to be dealing with these strong detergents expecially in a long soaking process. I think you'd be better off just using the salts, baking soda, and perhaps some dry skimmed milk if you're wanting to sooth tired, aching muscles. All the shampoo is going to add is bubbles. The added milk will make your skin as soft as silk.
Sorry to burst anyone's bubbles. :) Julia in Orlando, FL