OK, In my opinion an Oil Syringe makes things quite easy. These are made by people such as 'Draper' and can be bought for under a tenner.

Anyway, when you look at the front of the gearbox, you need to remove the Filling Bung, which is the top one on the front. Fill the gearbox until Oil starts to overflow out of there, then replace the bung. You must make sure the car is level when doing this.

I like to use a syringe, because the oil goes in under a little force which makes sure it gets into all the nooks and crannies.

Some 1 Litre bottles have an extendable nozzle which is fairly easy to use too.

They take around 3.1 Litres.

Personally, I like to run a gearbox in similarly to an engine. i.e. Cover about 200-300 miles, then dump the oil and change it, although this isn't practical with a gearbox that doesn't have a sensible drain-plug. I have had one added to mine on the very lowest point on the bottom. This makes it LOADS easier.