basically theres a drain and fill plug on your tranny, open the fill plug and top her off, if you plan on replacing all the fluid then first open the fill plug so that there isn't a vacuum and then the drain plug. it should drain about 5 quarts, or in your case 4, if you are low a quart, so make sure what ever you are draining into is big enough. Cap off the drain plug as it stops dripping and then fill her up through the fill plug. I personally just fill it up until it dribbles out of the hole a bit.
Notes: *Recommended that you do this on the flatest surface possible. Get jack stands for each corner if you can to make sure the fluid all drains, and fills properly.
*With an Auto Tranny, always drain it when the car, and tranny are cold and has been sitting for a while. When it warms up the fluid gets all up in the gears and such and does not drain very well at all.
*With a Manual Tranny, always drain when the car, and tranny, is warmed up. The hotter the tranny fluid the easier it will flow out when you drain it.