How do you flush a transmission?
You can take the car to reputable transmission shop and have
them flush it professionally.
If you want to do it your self... You can do it if your car has
an automatic transmission and a transmission cooler.
You need to take off the cooler lines and determine which one is
the intake (brings the fluid back into the transmission). Take a
clean bucket and put the intake line in it; put the other line in a
separate bucket. Fill the clean bucket with new transmission fluid
and start the car.
The transmission will begin to pump out the old, dirty fluid and
suck in the clean fluid. Make sure the intake hose is always
submerged; you do not want it to suck air. Run the car until you
start seeing clean fluid being pumped out.
After all of the fluid is changed, you need to change the
filter. You must do this or the new fluid will become contaminated
and there wouldn't have been any reason for you to have flushed the
transmission. You will lose a few quarts of fluid when you drop the
pan; you may be able to catch it and reuse it. After the filter is
changed, top the transmission off with clean fluid.