In IFR situation you can forge the essential control signals used by the ILS system without touching the instrumentation of the airplane.
There are two principles of airplane control: VFR (Visual Fight rule) when you can clearly see the track the plane should follow; IFR (Instrumental flight rule) when the visibility is poor or insufficient for proper governance. In IFR case the track of the final approach is determined by the signals of ILS (Instrumental Landing System). ISL signals are thought not to be forged because the antennas of the system are on the ground on well-defined positions and radiating steady radio signals. The airplane evaluates those signals, the LOC (Localiser) and GL (Glide slope) and calculates its own position related to the center of those signals. The LOC and GL signals conduct the plane to the touch-down point like a funnel. The field strength of the ILS signal is very weak on the plane because field strength weakens by according to the square of the distance. If you can generate and radiate ILS-like signals on board those will kick the original signal out and substitutes them with your signals. All you have to do: learn the GPS coordinates of the targeted/intended touch-down point, learn the exact GPS position of the plane and generating and radiating your LOC and GL signals. You only need a commercial GPS and /or GLONASS receiver, a laptop, a special (home brew) "ILS-modem" and two tiny (5 mW) transmitters. On the other hand you can emulate ILS signal on airfields without ILS support.