gear lube

I have read a couple threads regarding changing the gear/stator oil. Just looking for a bit more clarification. I know there are the drain and vent plugs, next to the intake grate. Maybe someone can help me with the proper procedure. Obviously the drain plug is removed to drain the old...

my 1989 sp runs super. but last fall I replaced the wear ring and impeller. when i started it on the trailer after the repairs the impeller scraped a bit at first but then it smoothed out quickly. then all of the gear oil dumped out the back! yikes...what did I do... or not do?