It's a pain in the ass!
If it's a small leak I wouldn't worry about it too much until you have the time and energy for it. You can top up the gear oil pretty easily. Heck, in a pinch you can pull the speedometer cable and squirt a liter down there... ask me how I know.
Anyway, I had to do this on my Jetta. It wasn't much fun, even getting to the point where you've got the axles out can be a PITA. My Jetta had such high mileage (596k kms) that the axles were actually worn by the rubber seal. Kinda like the RMS on high-mile engines. Anyhooo, I still have my homemade tools if you want to borrow them.
Good DIY guide here:
http://www.brokevw.com/020flange.html
Keep in mind you won't be standing upright working on parts on a workbench. You'll be laying on your back & side, saying BAD WORDS