I am assuming that you have a 722.315 euro transmission.

You attach a pressure gauge to the port next to the modulator. Disconnect the vacuum line. Drive around in D at 50 km/h (30 mph). Pressure should be 2.9 bar. If not, take off cap of the modulator and turn up or down (to the right is more pressure). 360º turn of the key is 0.4 bar adjustment.

If you don't have a gauge or if it is too much trouble, you can also use the method described by Bob. Adjust until the shifts at partial load are neither harsh nor slipping.

The vacuum system must hold vacuum, so replace the rubber socks, threeways, fourways and the cap of the modulator if suspect.