I had the same question about my 05 GT. The answer is:
Warm it up to operating temp (90 C, I think.)
Lift it up, keeping it level.
Pull the plug in the bottom of the
trans. It's not a drain plug, it's at the bottom of a tube that sticks up inside. Some fluid will spill out because it sloshes in as you drive.
Now squirt A/T fluid up the tube into the
trans until it overflows and runs back down. It will stop spilling back out when the fluid is level with the top of the tube.
I guess it's almost fool proof, but sure is going to be a PITA without a lift.