To avail a senior citizen discount on jeepneys, you need to present your senior citizen ID card to the driver or conductor when paying your fare. The discount is typically 20% off the regular fare, as mandated by the law. It's a good idea to check if the jeepney company has specific policies regarding the discount, as practices can vary. If you're unsure or have issues, you may want to consult the local transport authority or the official government website for more information.