To apply for the 20% senior discount in the Philippines, you need to present a valid Senior Citizen ID at the time of purchase. This ID is issued by the Office of Senior Citizens Affairs (OSCA) in your locality. Make sure to keep the ID updated and valid, as it is crucial for availing discounts on various services and products. For more details on application procedures and requirements in your area, consider visiting your local OSCA office or checking their official website.