Hello,
I assume that you are dealing with signals with small BW/(center frequency) ratio (so in fact sinusoidal signals that change phase and amplitude slowly).
The I component you will find by multiplying your signal with 2*cos(w*t) and pass it through a low pass filter to remove the out of band products. w = omega (2*pi*f) of center frequency
For the Q-component you use same setup, but you multiply with -2*sin(w*t). The Q-component represents the imaginary part in the complex plane.
Wim