I am not a transmission expert but the first thing you did wrong was not draining the torque converter at the same time. That is where most of the old fluid is. However that would not cause the problem you have.
The only thing I can think of that could have gone wrong is that on the neak of the filter there is a rubber sealing ring, sometimes when removing the old filter that ring stays in the transmission and will not allow the new filter to seat properly. Check the old filter to see if that seal is still on it.
I assume that with engine at idle and transmission in Park the fluid is at the correct level on the transmission dipstick?