You can also disconnect the return line at the tranny and pump it out. This processes came from a Ford tranny engineer.
Purchase 20 qts of ATF, Mercon only. Do not use Mercon V.
Drain the pan.
Refill with ATF, about 7 qts
Run truck till oil stops flowing. (Pump it out)
Refill with 6 qts ATF, 13 quarts total at this time then run truck till oil stops flowing again.
Put tranny back together then refill with 6 qts of ATF. 19 qts total at this time
20th qt of ATF to top it off full but not overfull.
My method.
I've been told it gets close to all of it changed. I did mine by draining pan, T/C and then pumping last 2 qts from cooler and feel I got as close to 100% as you can get.
Jim