How to wash a bike helmet properly?
Rather than any complicated method or products, I do the following:


Take it with me when I shower.
Lather up hair with my preferred shampoo.
Loosen straps and fittings, place helmet on head and move and squish it around a bit to distribute the shampoo.
Rinse
Repeat.


Shampoo or body wash works great.  Then I just clip the straps over a line to hang the helmet to dry.