Drop a cooler tube, aim the open end into a drain pan. Start the engine. When the fluid starts to sputter, shut off the engine.
Drop the now nearly-empty pan, clean the pan and magnet, replace the filter, reinstall and seal the pan with a good gasket--which might be the reusable one already on the vehicle. Torque pan bolts as needed.
Dump five quarts of fresh fluid down the dipstick tube, open about ten more quarts and place them within easy reach so you can dump them quickly down the dipstick tube.
Start engine, watch the fluid coming out of the cooler tube and dump the opened quarts of fluid into the trans as fast as it will flow out of the funnel down the tube. When the fluid shooting out the cooler tube looks virgin-new, shut off the engine. Reconnect the cooler tube. Start engine, adjust fluid level as needed. ("Full" when HOT, a little less than that initially.)
Pack up the old fluid for recycling. Most of the time, I pour it from the drain pan into the empty bottles so it doesn't slosh out of the drain pan and mess up the bed of the truck, or the carpet in the trunk of the car. In my case, it becomes fire-starter in the winter; but before I had a fireplace it went to various recycling centers; or to shops that had a waste-oil furnace.
Done. Feel proud about flushing 95% or more of the fluid from your transmission. Open and enjoy a celebratory beverage.
When dropping the cooler tube, pay attention to normal fluid flow--so that you aim the correct tube into the drain pan. In some cases, I've gone so far as to make a short, temporary tube to plug into the outlet port of the cooler, to aim into the drain pan. That way I flush the cooler, too.