Most gearboxes have a filler on the side which is positioned to give the correct level (the same applies to filling rear axles).
Filling requires either a 'syringe' or a plastic bottle with a flexible tube:-
http://goldwingdocs.com/Images/HowTo...ear-Oil-13.jpg
The sort of one litre bottle that hypoid gear oil comes in (or used to) usually came with tube attached. To fill the gearbox squeeze the bottle and wait until the oil runs out of the filler hole.
Don't forget to feel around inside the drain hole after you've drained the oil to check for swarf (and loose teeth!). It's best to drain the oil when it is warm, but you might not have that luxury, in which case it's worth pouring some paraffin (or petrol if you haven't got paraffin) in the filler (after you've replaced the drain plug!) to sluice out any debris that has settled on the bottom.