I've got a good friend who owns a carpet cleaning company.
Only use those things on carpet YOU won't have to use (i.e. selling a house) I've seen first hand what it does to carpet over time.
If you insist on using it; be SURE to clean once with soap, then fill the soap container with CLEAN water, and clean it AGAIN to get all the soap out of the carpet. Then be sure to let it dry 100%. It's best to clean during the summer when your a/c will work as a dehumidifier. I remember when my grandmother got her carpet replaced after being cleaned about once a year by those red rug doctors (by me, I would use soap, then use clean water, then let it air dry 100%) and the carpet backing and padding was all nasty simply because those things do NOT have enough suction to get enough water back out of the carpet
during winter you're better off paying someone $140 to clean it.