I have the gearbox manual, but it's not that helpful. The only thing it mentions is how to check the level.
I'm going to do mine soon and have a plan of how I **THINK** it should be done.
1. Put car on flat surface.
2. Squeeze under car and undo gearbox drain plug (this is obvious if you look underneath the gearbox). Make sure you have a container to catch the oil.
3. Once it is drained, refit the drain plug.
4. Undo the level checking plug on the side of the gearbox.
5. Refill with oil until it just starts to overflow out of the level check hole.
6. Refit level check plug.
7. Job done
Obviously this would be much easier if you have access to an inspection pit or similar so you can get under the car. You should try to keep the car flat to ensure the level is spot on.
I can post a link to the gearbox manual when I get home if you like.
Hope this helps