Okay, the first thing you need to do is check and make sure that your car actually needs Transmission fluid. This will require a new filler/level plug sealing ring whether you change out the fluid or not.
Disclaimer: As usual, all of this is at your own risk. Neither I, nor UB will take responsibility for anything that happens to your car by following this guide.
1) To check the Automatic Transmission fluid level, you must remove the filler plug from the transmission fluid pan (which is under the car). While you can do this without jacking up the car, its a little easier to work if you do. Please use jack stands if you do jack it up.
2) When you check tranny fluid, the fluid can't be cold, but it also can't be at operating temperatures, so drive it around the block and make sure the car uses all the gears. You'll regret it if you drive too hard (second degree burns anyone?), so take it easy.
3) Turn off the engine (This should have been a no brainer).
4) Place some sort of container under the transmission fluid pan jsut in case (its normal for a little bit to drip out anyways)
5) Unscrew the filler plug and recover the sealing ring.
6) With the plug out, restart the engine and shift the car into Neutral.
Fluid Levels should be up to the lower edge of the hole. If necessary, top-off the fluid until it overflows from the plug hole.
While you're here, don't forget to check the condition of the fluid. If the fluid is black, or a dark reddish-brown, or smells burnt, it should be replaced. <Link to fluid change thread coming soon>
7) Replace the filler plug using a new sealing ring. You'll kick yourself later if you don't use a new ring. Trust me on this one.
8) Put the car back in park, turn off the engine, and lower the car (if you raised it)
And you're done!