I seriously doubt you will find a 'canned' D&D procedure for your specific business. And your service will probably be different from typical IT connectivity services. Those are typically 2 way data connections and my bet is yours are more like a transmitter with the aircraft having a 'radio' of sorts which can pick up multiple radio stations. This, of course, a guess which I am a bit wary of because you say the plane has a modem. Modems are commonly thought of as 2 way data boxes.
OK - So, start out with an airline or aircraft manufacturer sending you a request for quote. From that you get their initial specs/customer requirements. Have you done, for example, a flow chart of the overall RFQ process? What departments are involved?
I assume you evaluate the aircraft electronic entertainment system and determine the integration requirements so that OK - They need a modem, they need an appropriate receiver, including at least 1 (probably more) antennas. I assume you are transmit only other than data handshakes and I assume there may not even be a traditional two-way data handshake in that your system is probably transmit only like satellite TV in a home in which the only 2 way transmission is over a phone line.
So - You need to know their hardware characteristics and interfaces. Your service is somewhat like designing an aftermarket automotive AM/FM/DVD/SatNav/etc.
I know you do really know what you do. All you have to do is document the process you go through to determine the aircraft infotainment system specifics and capabilities, and what outputs (transmissions) you provide.
Just a few thoughts off hand. I can not go any further because I don't know some basics which are probably company confidential.