Let me go one step more than telling you how.

That vacuum modulator is basically a valve... like your outdoor faucet....
when you go clockwise, you close off the water. In the car, you reduce the
vacuum to the transmission... making shifting more firmer.

Counterclockwise is opening up the faucet so more water go out... same thing
in the vacuum modulator... more vacuum and making the shifting more
smoother.

In any case, too firm is uncomfortable as it might snap your neck... just
kiddin... and too smooth will let your transmission slip more and wear out
faster... so you need to find the middle.

There you go!