There are internal modules you can get from Hobbyking by either Assan, Corona, FrSky or Orange that you connect them internally and they modulate the signal through the 2.4 encoder to transmit it. I would recommend either the FrSky as it has a very good protocol (ACCST) with telemetry even and the receivers are easy to find in the UK, or the Orange one which uses DSM2/DSMX, for which the receivers are dirt cheam from Hobbyking and you can also use it with the american made BNF planes as they all use that protocol... you CAN even put both the modules in and swap between them with a 2P2T switch.
Before you start, you must find 3 connections inside your radio: Vcc, GND and PPM. The Vcc is generally any 5V or there abouts (the modules permit a margin) point from the radio. The GND is easy as this you can even get straight from the battery negative connection if you wish. The hard one would be the PPM signal, which if the radio has 2 boards, one as transmitter and one encoder, then it's "the third wire" that connects these. If it's all on one board, then you're going to have to dig around for info on the net and see if anyone has isolated the signal carrier with an osciloscope. One question that might help, although being 27mhz I doubt it... does the radio have a trainer port of any sort? What make and model is the radio?