I normally just drop the pan, replace the screen and replace the gasket. I notice Ford tried to have a reusable gasket but mine ended up breaking apart.
Typical amount of fluid is 4 - 5 quarts. Use exactly what is spec. out. If you notice more than a drop of garbage on the magnet consider reducing the time between fluid/ screen changes. If you are asking "what magnet?" there is usually a magnet stuck to the steel pan to collect metal bits from wear and tear. Bad shops remove this to speed up failures and hope for more work. Find a replacement magnet in a hardware store or independent car parts place. Put a coating of grease on the top of the gasket (where it seals against the transmission) to make removal easier next time.