Get a really big toilet
There are two other methods:
1. One of the fluid cooler lines is split and they insert a bladder tank in series. This is the same as the water well expansion tank that has the bladder inside seperating the air charge. One side is filled with new fluid. As the old is pumped in by the transmission, new is exchanged.
2. Disconnect one of the cooler hoses. Drain pan, change filter, refill and add two extra quarts of new fluid.
Start engine, let idle, and stop it when three quarts is pumped out. Refill with three quarts and repeat. Continue till fluid looks clean, reconnect hose and top off levels. Usually takes 15-18 quarts.