To do this procedure, you will need to find a strong, stable RTTY signal on the air. I suggest you attempt this during a contest weekend, or you can use the ARRL RTTY bulletins, or a maritime station such as FUF which runs RYRY test tapes almost constantly.
The first step is to find the station and tune it in by using an audio spectrum analyzer such as Freq v5.1. Tune it so that the low tone occurs at 2125 kHz. At this point close the analyzer program and run your TNC software. At this point text should be scrolling across your screen. Take a look at your tuning indicator...is it noticeably off to one side...or maybe not even gyrating around the center?
To adjust, turn the trimpot R212. This trimpot is the small blue pot right behind the tuning indicator and to the right of the ribbon cable coming from the front panel of the TNC. Continue to turn this pot until the center two bars of the tuning indicator are lit. When they are, or at least the lit bars seem to gyrate around the center, then the tuning indicator is correct.