The way I normally change tranny oil is: remove the fill plug (the one on the side), place drain pan under the drain plug, remove drain plug. While the tranny is draining, remove the shifter, when the tranny stops draining (can take quite awhile if the oil is cold), put the drain plug back in. Pour a quart in, let it settle, then another 1/2 quart, stick finger in the fill plug, if you can feel the oil level then you're close, when it starts coming out of the fill hole then it's to the top.
Just remember to use the right weight oil, I have found living in Indiana, 80w90 is too thick and doesn't work too well when it's cold.