If you are draining the tranny using the drain plug, you need to put the plug back beforte adding any oil. Remember that this method only gets about half of the oil out. The oil in the torque converter remains. I've done it this way and even half new oil made an improvement in the way the tranny ran.

A better way to flush the tranny is to place a tube on the upper radiator oil cooler connection and run it down to a container like a milk jug. Don't mess with the tranny oil pan drain at all. (I think this may have been what you planned to do, right?)

When I did it this way, I started the engine and let it run until air bubbles started to show in the transparent vinyl tubing I used. I then shut off the engine and replaced the oil which had come out, about 2 quarts. I continued until I had run about 10 quarts through the tranny and the oil coming out looked clean and red.

I deliberately underfilled the tranny at the end to avoid adding too much oil, a bad thing to do. After I reconnected the oil cooler line, I drove the car to warm up the tranny oil then filled the oil to the full mark on the dip stick.

I need to do this on my daughter's car soon.