All you have to do is disconnect both hoses from the cooler (radiator) and place them in jugs to catch the fluid (messy - have a drip pan). Idle the engine for a moment and find out which one spits out fluid. This will drain the transmission cooler also. Then you can either leave the hoses off and collect what comes out of the output hose, or reconnect it to the cooler and catch what comes out of the cooler on the return line connection (then you need a second hose to direct the fluid from the cooler into a jug.
It is not necessary to flush the transmission, but I am just anal about some things. Replacing what is in the pan periodically is also fine. I just don't have a feeling for how often is acceptable when you are leaving 75% or more of the used fluid in the box.
In my case, I replaced all the fluid with (expensive) Mobil 1 ATF - hoping to avoid this for a long time. Synthetic fluid is very heat stable and should last a really long time in the tranny, where air oxidation is not as significant a degradant as it is in the engine to engine oil.