It’s an easy adjustment. The procedure from the 1990 service manual:

1. Disconnect battery (your call)

2.Raise car up and place shifter in neutral.

3. Get under and loosen the nut that the cable attaches to on the side of the transmission(not trans. pan bolts!!).

4. Rotate the shift lever(on transmission) clockwise (towards the front of the car)

5. Shift transmission back two clicks to neutral (may be hard to find on the first try).

6 Rotate through the detents and see how it feels.
You should check that the trans. pan bolts are tight before adjusting it since the cable mounts to them as was pointed out earlier