These instructions are for a late model (1993) Cabriolet with a 5 speed manual transmission. It takes 2 litres of fluid. Other models of Mk1 Golf are different in that they may take less fluid and be filled from the speedometer cable drive rather than a dedicated fill point. I used 80w-90 oil.
There are 2 17mm Allen bolts, 1 for drainage and 1 for filling.
(Excuse the oily, drity transmission in the pictures - there was a leak from the oil pressure sensor at the time.)
Jack up the front of the car and set it on jack stands. Remove the transmission-side wheel - this will reveal the fill 17mm Allen bolt. It faces horizontally pointing out from the car.
Use the 17mm hex drive to remove the fill bolt. You should try to remove this one first as if it's stuck you won't have already drained your transmission oil in made the car immobile
Mine was pretty stuck - I had to use the electric rattle gun to loosen it.
The drain plug is located in the centre of the transmission directly under the car. It's the same 17mm Allen bolt. Removing the drain plug was also difficult and I had to use the rattle gun again. Let the transmission oil drain. Be careful - it stinks.
I cleaned the drain plug and replaced it in the transmission and prepared the new fluid. My car takes 2 litres of fluid so I used a measuring cup to measure it out. It's a real pain to fill the fluid from the horizontal angle of the fill plug - I ended up using a turkey baster funnel and spare length of power-steering fluid pipe, but some new transmission fluid comes in specially designed for this purpose.
Fill the transmission with the required amount, or until it starts leaking out the fill hole.