By "serving pressure", he means pick a a carbonation level, then use a calculator like this:
to determine how many PSI that is at your system's temperature.
And yes, you'll need to empty the headspace in order to lower the pressure after you turn down the regulator. Your regulator won't dump that extra CO2 back into the tank...
You can also just pour a few beers instead of venting, that'll help bring the pressure down
A lot of us just use the "set and forget" carbing method, where you leave the keg at serving pressure for a couple weeks to slowly carbonate. If you aren't in a hurry, it's the most foolproof method.