Make sure you have the correct bit for the drain and filling plug as if you dont you risk rounding it off very easily.
The gearbox drain plug is found underneath the gearbox lol, it has a square shape plug. The filler plug is located on the front of the gearbox and again is a square plug, it is right by the gearbox engine mount.
As Rishi has pointed out, fill the box until it starts to trickle out of the filler plug hole, this normally takes just over the 4litres to fill.
I use the Millers 75-90W gear oil (not the LS oil) which i find to be very apt.
Dont forget to change the transfer box and rear diff oils to as these degrade just as much. For the rear diff try to clean out as much of the old oil / dirt / metal
with your fingers as obviously the less the better.
Hope that helps.
Ant.