Use bottle of hydrogen peroxide that is 1/3 full. If you have a full bottle, either pour some of it out, or use another dark colored bottle. Any clear or semi-translucent bottles will break down the hydrogen-peroxide neutralizing the cleaner.
Fill the bottle the remaining 2/3 of the way full of water. Add 5 drops of lemon oil. Shake lightly.
Liberally spray the carpet with the cleaner mix. After the mix has set for a min or so, use the scrub brush to loosen the fibers. The yucky stuff moves to the surface. Use a rag an wipe the carpet, blotting out any liquid.
Repeat a couple of times until your carpet is clean.
You can also use this cleaning mix on upholstered furniture.
Be careful on some carpets. It did not affect our car carpet, but hydrogen-peroxide has been known to lighten darker flooring. Test it behind a door to make sure that it won’t lighten your spread.
If it bleaches, dilute with more water.