Hi Anthony you'll need to remove the gearnob, leather and rubber gaitors, get the car on a lift and remove the exhaust. Put a gearbox jack under the rear of the gearbox and take the strain of the box and engine. undo the 2 hanger bolts to allow the rear gear box to be lowered when ready. Then undo the engines upper mounting and then back underneath to lower the box down ready to remove the gear selector shaft and frame. Tap the cottor pin out of the selector sleeve and remove that off the shaft. Now you can get to the gear selector shaft oil seal, using some kind of hook tool you can pull it out and slide the new one on, tapping in it into place with a piece of tube or deep reach socket.
good luck mate, col