Needless to say, the first thing you need to do is vacuum thoroughly.
Now, you are ready to prespray the carpet. Some users like powdered pre-sprays and others like liquid pre-sprays when cleaning carpeting. The difference between your regular pre-spray and professional pre-spray is that the professional prespray have more than just one ingredient. A professional carpet cleaning pre-spray will remove oil-based stains, organic stains and protein stains.
Mix your prespray according to directions. Using a pump-up sprayer or a “hydroforce” apply it evenly to the carpet. Do not overspray. Overspray causes the stains to go even deeper in – making it difficult for you to extract.
After you have presprayed, get your machine ready to clean.
Add a little bit of detergent to the tank containing the water. Without chemicals, no “real” cleaning ever takes place. Detergents also come in powder or liquid. Mostly carpet cleaning chemicals that are in the form of powder are more popular. Professional carpet cleaning chemical detergents have an advantage over the regular detergents as well. Professional carpet cleaning chemicals contain anti-resoiling agents and are lower in pH, so the carpet is left in a better condition than just using water or low-quality detergent.
Now, thoroughly extract the carpeting using your machine. Do not over spray, rather repeat dry strokes to remove as much water as possible.
Usually, at this point, if the carpet is not as dirty you are close to being done and ready to dry.
If the carpet still has spot and stains left, you will need to know what would need to remove the different types of stains that are left behind.
The most common stain left behind is usually a rust stain. To remove rust, you need a very low pH solution to loosen it and then you rinse it out.
Then you have your oil-based stains. Gum, nail polish, grease and such. To remove these you need a product that contains d-Limonene. d-Limonene is the juice from the skin of the orange. It is a solvent that loosens up oil-based stains. There are two types of solvents. Volatile and nonvolatile. What you need to know is that volatile solvents are needed to remove the residue that the nonvolatile solvents leave behind when removing oil-based stains.
After all the stains are removed, a good carpet cleaning brush can be used to groom the carpet so the carpet fibers dry evenly.
If you have come so far and have had good results, it is time to protect the carpet from future stains. There are many products like Scotchgard and Teflon that form a coating to prevent stains. I actually prefer carpet protectors that change the surface tension making them repel dirt.
There is a lot more too it, but this is what you would need to know in a nutshell. The IICRC offers a 2-day Carpet Cleaning Certification Class that will teach you a lot more.