The specific "how-to" depends on the model of iTrip, as there have been lots of different types (just as there have been lots of different types of iPods), but the concept goes like this:
1. Connect iTrip to your iPod.
2. Tune the iTrip into an EMPTY radio frequency. You need to find a radio frequency that normally sounds of static, because you're going to use that frequency for your own mini radio station (ie: Your iPod).
3. Tune your FM radio into that frequency.
Manuals and support for specific models of iTrip can be found here: www.griffintechnology.com