Just add water.
A bar of wool wash works just like a liquid wool wash, only instead of having the water pre-mixed, you add it yourself. You simply lather the bar in your hands like a regular bar of soap, let the lather fall into a bowl or tub of warm water, and submerge your knitted item. It's that simple.
Additives: Lanolin
Other ingredients: Saponified oils of coconut, palm, castor and olive
Here's Tuft Woolen's tutorial on how to use a wool wash bar.