Here is one way to flush out your tranny - however if your fluid is very burnt there are most likely internal parts damaged and you would be better off with a tranny rebuild and a new torque converter. This uses the transmissions own pump to pick up fresh fluid from pan and flush out the old stuff.

Step one - determine which cooler line is the return line to the transmission and disconnect it.

Step two - replace tranny filter pan and gasket.  Refill pan with new fluid.

Step three - place one gallon container under disconnected return line.  Run engine until container fills with fluid and turn off engine.

Step four - refill transmission pan with four quarts of new fluid. Remember to run tranny through all gears to flush all old fluid from valve body.

Repeat this until fluid runs clear out of return line.