How do you wash dreads?

You need to wash them either with a specialized shampoo, or one that you know contains only natural ingredients.
You don't need to wash all your dreads, just the scalp. Then, as you rince, the shampoo will wash the surface of the dreads. The reason for doing it that way is that you don't want soapy residues inside your dreadlocks.

technically you dont need a specialized shampoo or shampoo at all
you just need a residue free soap
dr bronners diluted 12-1 is good, but most prefer the baking soda deep clean (see links)