I've searched for a while and I didn't find much on what is the best way to do it. I have a 2003 F150 with a 4R70W with no drain plug.

Denny

 

I followed this thread when I did mine. I added the drain plug to make future drains much easier. Basically you drop the pan (very messy), change the filter, clean the pan (and install a drain plug if you want) and bolt it back up. you need around 4-5 qts with a pan drop. Since you only get 1/4 of the fluid in the transmission, you need to do several pan drops (or drains) to get most of the fluid drained.
The thread linked to is for the 4R75W, which is just an updated version of the 4R70W.
After the first change (I did the filter and added the drain at the same time) I noticed some better shifting (had a "thud" when shifting between 3 and OD that went away). I have done 2 more drains since that one (one at 500 miles, the last 1000 miles after that). I am coming up on 10,000 miles since that, so am planning on draining the pan again next oil change.

There is also the option to use the cooling lines to DIY. HERE is a site that describes that.

 

Thanks I'm going to do the cooling line change to get most of the fluid out with a filter change and add a drain plug. This truck is new to me with 121K miles so I will change all the fluids for a clean start.

I was looking for a beater truck to save wear, tear and miles on my pulling truck and came across this very clean 03 F150 that will be my ever day truck (not a beater) when we are home in the summer.

Denny