The plug your looking for is a 1/4" plug on the right side of the trans housing. It's at the back corner close to the same level as the PTO shaft and axle.
The 1" fill plug is on the top deck next to the shifter. It takes a 1/2 ratchet to unscrew.
You fill the trans until it comes out the 1/4" plug.
The drain plug is at the center/bottom of the trans housing.
Unscrew the drain plug and get a sample of your existing fluid. You may get a shot of water then oil. If it's clean top it off. Not clean will look milky, brown, goopy crap that may not drain at all. You may need to poke a through the crud to get it to drain. Last one I did took several days to drain and stop dripping.
If it's bad I drain it and dump 5 gallons of diesel in. Then go for a easy no load spin around the yard and let it sit for a day. Drive it around again and then drain and refill.
Takes about 5 gallons, I use 85w-140 gear oil. Book calls for straight 80w-90.