Go to Home Depot and get one of those heavy gage plastic "stucco" mixing pan to catch the fluid. Place it under the transmission and then loosen the pan bolts and let it drain. You'd have to guess-timate how much drained...at least 4 quarts. Retighten the transmission pan, fire the engine up, shift from "drive to neuteral to reverse several times to warm the tranny then check the dipstick. Add accordingly.
Good luck.