The oil drain plug on a 1988 Ford Tempo GL is located on the
bottom of the oil pan, which is underneath the engine. To be precise, it's on the
lower passenger side (right side if you're looking at the front of the car) of the oil pan. You'll need to get under the car to access it.
Remember to use a proper sized wrench and drain pan to catch the old oil. Always refer to your owner's manual for specific instructions and torque specifications.
