The shampoo foam itself is what you will use to clean the upholstery instead of the soapy water. Upholstery, especially when made of seat cloth or velour, has a tendency to look dry even after being soaked. As such, it is very easy to over-apply cleaner if using soapy water or a spray-on shampoo.
The foam in your bucket will likely die down as you work, so you may need to re-agitate the soapy shampoo water periodically to create more foam. If necessary, you may even mix in additional shampoo. For leather upholstery, do not use very alkaline soaps and they’ll stain the leather.
