A full out flush isn't necessary, but it can be a good thing to have done every 60 - 75k miles. But a simple fluid change is all you really need to do.
The gaskets are re-usable rubber, so as lon as it isn't dry rotted, cut or broken, you can just clean and put the old one back. Filters tend to last a long time between changes, but I would take a look at the condition of the fluid before you go changing the filter.
If the fluid looks clean, chances are the filter is fine.
A fluid change is pretty easy. The drain plug is a two peice plug. The larger outer part is the actual drain plug, but it has a smaller part in the middle which you use to check the fluid level. The outer part has a tube which sticks up from the bottom of the fluid pan. The top of the tube is the proper level for the fluid, any excess fluid in the transmission will drain out when the smaller plug is removed.
The car has to be mostly level when doing this so all 4 corners need to be on jack stands, unless you're of the lucky few who have access to a lift. You remove the drain plug and let it drain, then put it back in. Take out the smaller plug in the middle, and remove the round plug in the end of the tail shaft housing. Use a fluid pump to pump Mercon 5 transmission fluid into the trans until it starts to flow out of the tube. Then start the engine and shift to reverse, then drive, then back to park. Add fluid to the pan until it runs out of the plug. Then put both plugs back in their respective places and you're good to go.
