To find it:
1. Identify your transmission: Check your owner's manual or the sticker on the driver's side doorjamb. This will tell you the type of transmission (e.g., 3L30, 4L60-E).
2. Locate the transmission: The transmission is usually located towards the rear of the engine compartment, beneath the car.
3. Look for the pan: The transmission pan is a relatively large, typically rectangular or slightly trapezoidal, metal pan bolted to the bottom of the transmission.
4. Find the plug: The drain plug is typically located on the lower edge of the transmission pan. It's usually a magnetic plug (to catch metal particles) and is often a slightly smaller bolt than the pan bolts. It may be towards the back, front, or center of the pan.
Important Note: Since you're working under the car, use jack stands for safety. Always consult a repair manual specific to your vehicle's year, make, and model and transmission type before attempting this procedure. The manual will provide accurate diagrams and safety precautions. If you're unsure, have a qualified mechanic perform the drain and fill.