You can never empty a transmission once it has been filled ... unless you are willing to totally dismantle it. Too many passages and crevices that will not drain ... not to mention the torque converter. I will add a bit to what is already posted:
1. Disconnect return hose to tranny, place in container. Crank engine while tranny is in P ... stop engine as soon as fluid flow begins to stop (don't tarry ... stop immediately!!). Measure/record carefully the quantity of fluid removed at this point. NOTE: Caution ... do not shift though the gears during this step ... or until "test driving"!
2. (Optional step). Drop pan and clean. Replace filter.
3. Refill tranny with fresh fluid ... put in, at least, two quarts more than was measured in step "1". Crank engine while tranny is in P ... stop engine as soon as fluid flow begins to stop (don't tarry ... stop immediately!!). Notice clearity of waste fluid stream.
4. Repeat "3" until waste fluid stream appears new.
5. Reconnect hoses. Refill tranny with new fluid ... with almost (minus a 1/2 pint) as much fluid as was recorded/removed in step "1". Test drive, check for leaks. Top off properly once tranny fluid is warm.