Once a month, I add a cup of Distilled White Vinegar and a cup of baking soda during the HOT cleaning cycle. I pour them directly into the drum. Then, I add about ½ cup of vinegar and ½ cup of baking soda into the detergent dispenser.
Homemade Carpet Shampoo - revised the recipe as follows for use in regular size Rug Doctor: 2 Tbls. Baking Soda, 2 Tbls. Borax, 2/3 cup White Distilled Vinegar. Be sure to read my review in the comment section.