Clean the couch thoroughly with a vacuum to remove any dust and hair on it. If the couch is quite dirty, you can clean it with the upholstery attachments on a carpet cleaner. Rinse the couch after cleaning to remove any residue, and wait for it to dry before proceeding.
Set the couch on a tarp, drop cloth or cardboard to protect your work area. Open the windows or work outside if the weather is nice.
Take the cushions off the couch, shake the paint can, and spray the paint on the upholstery using a slow, even motion. Then paint the cushions, and set them aside to dry.
Leave the couch for 72 hours for the paint to set before using.