I've got a 1.9 td which may or may not be the same as yours.
If you crawl under the car (make sure it's propped up safely) you'll see the drain and filler holes are on the rear of the gearbox, one above the other. You'll likely need an 8mm square drain plug key, and you'll need new copper washers for both plugs.
It's a real pig of a job to refill the box with oil unless you've got some sort of remote filler. I used 500ml bottles (you need 1.8l) and some extreme contortionism (and some swearing).
