Get a sump plug set (cant remember which exact size but its a square one)
Under the passenger side of the car you will find the gearbox
at the passenger side of the box near the front there is a sump plug (might need to take the wheel arch out to get to it properly but i cant remember)
Undo that and keep the washer (just like your oil)
once its empty put it back in..
above that, toward the top of the box there is another sump plug, take that one out..
On the top of the box there is a small breather pipe (Can be some debate on this) but pour your oil in there..
Once it starts to drip from the top sump plug, stop adding oil (thats a level)
Put the plug back in, put the lid back on the breather..
Its full

nice easy job but it stinks lol...
as for oil, Citroen seem to have the cheapest and iirc its Total 75W80? and its like £6/litre from citron
According to haynes its 2 litres to fill the box