no pictures, but there's a drain plug at the back of the gearbox (need a new copper seal), and you refill it via the plastic 'wingnut' on the front of the gearbox (behind the radiator - need to remove airbox). there's a separate seal behind this wingnut, but should be OK to reuse
they take 3L 75/80 GL5 spec gearoil; easiest to refill using a short length of hose pipe/similar - with car level, refill until it overflows from the filling aperture
check for leaks (particularly from gear selector and n/s driveshaft) and make sure engine mounts are good (worn mounts will encourage oil leaks)
ed. as 172Ben