In a nutshell, you're adjusting the idle current (bias) on a warmed up tube with a trim pot.
Voltmeter negative probe to the negative speaker post, voltmeter positive probe to the positive port for the tube and you adjust the current at the trim pot with a plastic handled screwdriver to whatever mV is specified by the component manufacturer.
The voltage setting (bias) will drift with both tube age and input voltage from the wall/power conditioner.
Edit: I forgot to mention to make sure that the preamp volume is turned all the way down. I usually switch the selector switch to a setting which isn't in use just to be safe.
Think of it like setting the idle speed on a carburettor and you've got the idea.
