I just converted my '99 5.2 1500 with tranny cooler last week. You need a 5/8" ID clear plastic hose (10 feet), 14 quarts of ATF+4, and a clear plastic bottle.
1. Change the filter
2. Adjust the bands
3. Install cover
4. Fill to proper level with new fluid.
5. Attached the hose to the output side of the cooler (low hose on my truck).
6. Place the other end of the hose in the clear bottle.
7. With wheels caulked and good emergency brakes applied or all four wheels on jack stands (my favorite).
8. Place the plastic bottle where you can see it fill when you start the engine. When you start the engine and place the truck in drive the tranny will pump fluid from the converter through the cooler and into the bottle. Stop the engine after each quart and add another quart. After 12 to 14 quarts, the fluid in the tube should be clean new fluid. DO NOT OVERFILL THE TRANNY!
I like this method over a back flushing because I did not run the risk of sending varnish and crap back through my tranny and doing permanent damage.
Let me start by saying that what I know about transmissions would fit on the back of a postage stamp. But it is my understanding that a flush is not good for older high mileage transmissions that have not been flush on a routine basis. Since I just purchased the truck Ebay and I do not have a maintenance history I was not going to take a chance on flushing varnish and debris into the transmission and causing permanent damage leading to a complete rebuild. Some people have had success back flushing and other have destroyed their transmission. My goal was to replace the current fluid with fresh synthetic fluid without risking damage so I used the method above and I am satisfied with the results. I agree that the method I used does not get 100% of it out but changing 14 quarts is better than changing just 5 and did not risk forcing crap back through my transmission. These are personal choices. My Ram 1500 replaced a 1994 Grand Cherokee whose transmission never gave a single day of trouble for over 280K miles until I let someone talk me into power flushing it. The transmission died at just over 300K. Good luck with whatever you decide to do.