Apply 1 tablespoon of rubbing alcohol to the pen ink stain. This will help lift the ink, making it easier to clean. Let it air-dry for an hour.
Pretreat the stain with liquid detergent. Rub a small amount into the stain with a clean cloth.
Dip an old toothbrush into soapy warm water and clean the ink stain vigorously for several seconds. The stain should slowly disappear with hard pressure from the toothbrush.
Wipe the surface with a damp cloth and allow the couch to dry before enjoying it again.