Wet a towel with warm water. Add a drop or two of dish soap to the wet towel. Blot the spilled henna. Do not rub the henna; this can make it spread to other parts of the carpet.
Use a dry towel to soak up the water on the carpet. The towel picks up some of the henna along with the water. Wet the stain again and repeat this process until you cannot remove anymore henna.
Heat milk in your microwave or on the stove top until it is warm. Dab the warm milk on the henna stain and wait for 30 minutes.
Apply laundry detergent to the stain. Wipe up the detergent and milk with a wet towel. Repeat the process, if necessary.