Hi Mal
The basic concept of what you are doing is ok, but there are many considerations in carpet cleaning that you have to understand.
Pre vacuuming is good practice, Prespraying and brushing in the 'appropriate' cleaning agents is good practice.
May I suggest that you leave out the use of detergents in the tank of the machine and use either a plain water rinse, or an acidic rinse if required to neutralise any alkalinity within the prespray used.
Some people will agree with this others won't , but you need to make up your own view on this , and the only way that you can do this is by going on professional training and listening to the different veiws given on the forums.
I consider it like washing your hair , you apply the cleaning agent and the you rinse the cleaning agent away along with the soiling. Do you rinse off the shampoo in your hair using another shampoo?
You also need to learn to recognise the various types of carpet and fibres out there in order to select the appropriate cleaning agent, and method to employ. There are quite a few problematic carpets out there and by recognising them first off you will prevent many problems in the future.
I hope that helps
Good luck with your new enterprise.
Regards
Robert Olifent