Here is what I did if I need to flush either the steering or the transmission.

Just get a small T and solder three sections of 3/8" OD tubing onto it like the picture. Then you can splice it in between the return line with the compression union fittings shown there. The valve is the ball valve you use under the sink. These ball valves have Teflon seals that's perfectly OK for ATF.

Then just run the engine with someone turning the steering all the way right and left, alternately, and just drain one pint, fill one pint, ...... until you are satisfied.

Afterward, you can just shut off the valve and leave the whole thing in.