If its a MTX its easy! I did it three weeks ago when i did my headers. These is what i did:
1. Unbolt drain plug on the bottom of the trans
2. Let it drain
3. Unbolt fill bolt/hex
4. I bought three quarts of Royal Purple, 4 Oz. of Ford Friction Modifier and 1 quart of something cheap
5. Basically took the cheap stuff and put it into the trans with the drain plug out and just watched the fluid till it was nice a clean
5. Put drain plug back in and put in the good fluids
6. When you do it, the fluid should be right up to the fill hole (As of the How-To somewhere here)