Buy three quarts of your favorite trans lube and a funnel with a long tube.
Crack open fill plug. Mostly to make sure you can get it open.
Remove drain plug.
Remember that you didn't slide the drain pan under there first, and now have 2.3 quarts of nasty old gear lube on your concrete.
Replace drain plug.
Remove fill plug.
Fill trans until fluid comes back out the fill hole. Should take about 2.3 quarts. It's easier if you use the funnel with the long tube.
Replace fill plug.
Clean up mess.
Drink beer, and brag about another job well done.
