1st, don't be scared, it's really no different than changing engine oil !
With 100k+, drop the pan and change the filter (most just pop into place), reinstall the pan (gasket is reusable) and fill with 4 quarts or so until it shows full on the stick (I'm assuming you have an ATF fluid level stick)....then DC 1 cooler line and run the hose into a 5 gallon bucket, have a friend fire up the engine and when you have dumped about 3-4 qts, shut off the engine & refill.....the owners manual will have the capacities, but when you have pushed that quantity out....your change is done!
Member: Never trust a person over 40 who drives a Chevy club
Flatheads ain't so bad!
Certified backyard mechanic I & II