Not that hard, just the interlocks to detect the vehicle is in park or neutral before enabling it, and then switching the ignition and accessory systems on in the correct order. Most modern vehicles though have the ability to do that built into the BCM already, you just need to turn it on, and it will start the engine, though it will not allow the vehicle to be shifted out of park until you either have the key in the ignition, to allow immobilisor to communicate with the valid key, or the keyless entry does the same. If it is a modern vehicle you are stuck with letting the agents do it, or getting the manufacturer kit, but for older vehicles with no immobiliser (all metal key with no small chip in it) it is simple to install, just need to tie into the gearbox position sensor switch, and connect the appropriate power relays to the ignition switch and interlocks.