There are two plugs on the bottom of the transmission. the one lowest is the drain plug, the higher one is the fill/check plug.
Get the car up to operating temperature, put the car on jackstands and make sure the transmission pan is level. Pull the fill/check plug out. A small amount of fluid should start leaking out. If no fluid comes out you should add fluid until it starts to leak out. Reinstall the plug and you're good to go.