This was done on my Y21 gearbox.
Things needed:
2.5 litres gearbox oil (75w90) Honda originally used 10w40 engine oil which would be fine.
17mm spanner
3/8" ratchet
1 x 20mm copper washer
1 x 14mm copper washer
Jack
Axle stands
About 30 minute to do it and pack up.
Things handy to have:
Approx 2 feet length hose pipe.
Bucket
Big piece of cardboard
Start by jacking the front of the car and placing on axle stands, I also jacked the rear up to keep it horizontal and I have some drive on ramps to rest the car on.
From underneath you will see the drain plug (Red) and filling plug (green) on the drivers side of the gearbox:
Shove the cardboard under the car in case you spill some oil. Start by loosening the filling plug withthe 17mm spanner, you will be able to check the current level when it is removed. if it runs out, there was too much in, if you can't feel it then it was too low:
Next using the 3/8" ratchet, undo the drain plug, place the bucket underneath to catch the old oil:
Let it drain until no more flows out.
Replace the drain plug and use the new 14mm washer to seal the hole.
I then moved the induction pipe to make it easier to see from above:
Next put the hose into the hole, I had 3 x 1 litre containers to use so put the spout into the hose and let it drain, be patient:
When it is at capacity it will drain out of the filling hole so have something to catch it. I guessed pretty much spot on so very little was spilt. Give the 'box a wipe and replace the level plug with the 20mm washer.