There's a few ways of doing it, but this always works for me.
There's two plugs on the gearbox - the lower one is the drain plug, and the one higher up (facing the N/S arch liner) is the level plug.
Remove the drain plug and let the oil drain out. I also remove the level plug at this point. Clean any crap off the plugs (and renew the copper gaskets if you wish) and replace the drain plug.
Remove the battery and battery tray. On the top of the gearbox there's a black plastic cap. Remove the cap, and fill the gearbox through this hole with 75W80 until it starts to trickle out the level hole. Replace the level plug and plastic cap. Job done.
The two plugs are 8mm square (same as sump plug), and I think the torque setting for them is 25NM. As for the oil, I'm told the best stuff for these boxes is what Pug/ Citroen supply -
Total BV 75W80. You need 2 litres.