You need a mobile (or HT) radio, a Windows PC, a SignaLink, and the cables for the radio to SignaLink interface.
1) The SignaLink USB is the interface between the computer and radio. The SignaLink has to be configured per the instruction on Tigertronic's website (i.e., move internal jumpers), and then use the SignaLinc's cables to attach the SignaLink to the radio. The SignaLink is hooked up to the computer with a USB cable.
2) The Zello software runs on a Windows PC. Within the Zello program, you have to go in and select the sound card that will be the Signalinc in the settings.
3) If you want to transmit from a mobile radio to Zello, you have to enable "Vox on Receive" within Zello (for audio in.) Since the soundcard is selected in settings, the Signalinc will trigger the PTT on the mobile radio and send the audio from Zello to the mobile radio.
4) My son also said to disable (mute) all Windows sounds, since those will be transmitted as well.