Here is how I do it:
1. Change the transmission filter, then refill.
2. Pull off one of the lines going from the transmission to the radiator (I think it is the top line on the radiator)
3. Attach a long piece of hose to that line and put the other end in an empty bucket.
4. Start the truck and start pouring transmission fluid into the funnel as fast as it will take it while the old fluid is flowing into the bucket. If the fluid is burnt, keep this process going until the fluid coming out is bright red. If the fluid coming out is already bright red, continue until the have put in a total of about 14 qts (3.5gals)
5. Turn off truck, check fluid level. If fluid level happens to be too high, start truck again and let more fluid run into bucket, then turn off truck and check again. If fluid level is low, disconnect hose from transmission line, reconnect to radiator then top off fluid.
A few words of warning: don't fall behind on the fluid you are pouring in otherwise the fluid level will drop too low and you will burn your transmission up in no time. So have the tops of the transmission fluid bottles at least loosened if not off all together ahead of time.
Just saying that is the way I do it, I am not responsible for damage to your transmission if this does not work for you!