Doing a proper transmission service on my early 70’s Ford truck is a piece of cake. But with newer vehicles it can be very difficult to service the auto transmission fluid for a DIY’er. Yours in not an uncommon complaint. Take a look at the factory service manual procedure. I expect you’ll see the car has to be level at the time the procedure is done. Otherwise you won’t get the correct amount of fluid installed, or the correct amount drained out.
The easiest solution is to have a shop do it of course. It’s even more complicated than just keeping it level. Like I say, take a look at the car’s factory service manual. Often the car’s computer is involved in the process. If I was going to try this myself in my driveway – not that I expect to, my cars are too old – I’d have to invent a way to lift the car enough to crawl under it, but keep it level. Maybe drive the front wheels up on ramps, then jack the back wheels and put the rear end high enough on adjustable height jack stands the car was level.