Blot the stain with an old towel to soak up as much moisture as possible. Do not rub the stain, as this may spread it to surrounding areas of the carpet.
Add 1 teaspoon of neutral laundry detergent and 1 teaspoon of white vinegar to a quart of cool water in a bowl. Mix well. Pour some of this mixture in a spray bottle.
Dampen the stain with the solution, using a spray bottle. Dip a clean white towel or rag in the solution and wring out the excess. Blot and gently rub the stain with small circular motions from the outside of the stain to the inside, rinsing and wringing the towel often, until the stain disappears.
Rinse with fresh water using another spray bottle and a clean portion of the rag, following the same procedure as above.
Let the rug dry completely, then vacuum it to restore the nap.