Must be done with the vehicle level.
on a 1.8 K series [XEi] or an L series [XDi]:
1 remove the front left hand wheel and support the vehicle - [ lower it so that the vehicle is level]
2 remove the padded splash guard - that will allow you to view the g'box
The filler plug is rearwards of where the driveshaft leaves the g'box.
The drain plug is on the lower edge of the g'box casing.
3 Make sure you can get the filler plug loosened before draining the oil!
Use a clean container to catch the old oil and have a good look at it - colour ... can tell you a lot about how the g'box health is - the drain plug has a magnet on it how fuzzy is it? The old oil will hopefully be a dark brown colour with no metallic sheen to it and there could be some metallic fuzz on that magnet - no chunks!
Oil: I use a Penrite oil - Transgear 75W-80 which equals LR MTF94.
drain plug torque setting - 35Nm or 25ftlbs
filler plug torque setting - 45Nm or 33ftlbs
PGI G'box capacity = 2.0 litres [ 2.2l from dry]
Good luck