First Jack your vehicle up by the jack points and secure the vehicle with the axle/jack stands. NEVER GO UNDER A VEHICLE SUPPORTED ONLY BY THE JACK!
Next we will locate and remove the filler bolt. In some cases the filler bolt will become completely siezed that heating it won't even work. Or the filler bolt may become stripped. If this happens to you, you don't want to this to happen with an empty transmission.
In the first image we see the bottom of the transmission looking up. The recessed bolt on the bottom is the drain. The square head on the side is the filler.
Loosen the filler by selecting the proper wrench (not an adjustable one!) and turn the wrench counter clockwise. If the bolt doesn't let go; spray the wd-40 on the bolt and let sit for 1 hour. Still stuck. Don't force it too much as it may break. You can use a propane, or MPS gas torch to heat the housing around the bolt and then gently apply pressure to loosen it. In the picture you can see I used the wd-40
If the bolt still won't budge. Stop. Lower the car and visit a shop for service in rare cases the transmission must be removed and taken to a machine shop.
If you get the bolt loose. Unscrew it, but don't remove it completely.