[JCD]

It's pretty much the same as setting the tightness on a wheel bearing.
It'll only tighten if there's room to.

I should mention that the previously mentioned adjuster bolt on the top is only a slight adjustment anyway.
The main bearing load is on the side where the steering shaft goes into the box.
There's a tightening ring that holds it in place.
By backing off this ring you can tighten up the adjuster plug.
After it's tight, back it off just a touch, then lock the ring.

You'll be surprised at the change.
It can remove a lot of steering slop.

The above method is for adjusting it on the vehicle, but if you're going to remove the box, it's best to clean out the old grease, and inspect the bearings.

J.