In high school I worked for a company that half our work was carpet cleaning. Is it wet or dry? In other words, are the using the buffer style or steam cleaner?
If steam cleaner, they should shampoo it first, then go over it withe steam cleaner. Once done make sure they put something under the legs of the furniture, so they don't stain the wet carpet. We used foil or wax coated butter pads. Once we were done, we would "rake" the carpet out with a carpet rake, so it was all going the same directions.
That was a great job in high school.