The following guide is an adaptation of a MIGWeb post by Topbuzz

The F28 has a locator hole in it. Pull out the gear selector shaft in the engine bay so that it is in the middle position, ie slide the shaft in and out till you find the middle position.

Then slide a 4.5mm drill bit of allen key down the hole. Still putting pressure on the bit, twist the gear slector clockwise, the drill bit should fall down. The gearbox is now in neutral.

From under the car, undo the pinch bolt, you need to move the gearstick and gearlinkage individually to set them up.

If the car is later than 1993, then life is a bit easier as the cage around the gear stick has a location hole in it. On < 1993, its not there, but still has the hole in the gear stick.

Slide a 5mm drill bit or allen key down there. But the 2 holes are different diameters, so there is still quite a bit of movment there. You may want to cable tie the gearstick pulling it towards the driver seat.

With both the gearstick and gearbox in neutral, you do up the pinch bolt. In theory you should have all the gears plus reverse.

If not, it may be down to two problems

1) 10 year old gear linkage, with lots of slack
2) rubber ring round gear stick neck worn down