I used a 4" funnel propped into the passenger side mount of my STi titanium strut brace with a short piece of scrap hose from the funnel on down into the filler neck. Draped a couple old rags around the funnel & filler neck to catch any spills.
Might want to warm up the new gear oil 1st, as it flows _real_ slow. I sat the bottles in the kitchen sink full of hot tap water for a half hour or so. Dry off the outside of the bottles before filling up the trans.
When doing the rear diff:
-Take out the filler plug (upper one) 1st before draining the diff!
-Have a long 1/2" drive breaker bar handy as the plugs can be in tight. I put the car on ramps to get extra leverage room and more stability than jack stands.
-Handy to have a pump...I used a cheap one from AutoZone that mounts onto the gear oil bottle. Took many pumps to fill it back up. ;-)
-I have the Primitive rear diff guard, so I used another piece of scrap hose jammed into the drain hole down to a catch can to minimize the oil caught by the diff guard when draining out the old oil.
-I didn't use any sealant on the plugs, just a bit of anti-seize on the threads...no leaks seen.