Waveform samples and I/Q components
Assume that signal that comes to you through your antenna is:

$$ s_r[n] = I[n] \cos(\omega_0 n) + Q[n] \sin( \omega_0 n ) $$

which is a bandpass, modulated, narrow band signal where the components $I[n]$ and $Q[n]$ are the lowpass baseband in-phase and quadrature-phase components respectively.

Then in order to obtain the I and Q components from $s_r[n]$ you can apply the following just like the analog case:

$$I[n] = h_{LP}[n] \star ( s_r[n] \cos(\omega_0 n) ) $$
and
$$Q[n] = h_{LP}[n] \star ( s_r[n] \sin(\omega_0 n) ) $$

Where the lowpass filter $h_{LP}[n]$ is will have a cutoff frequency larger than the bandwidth of the signals I and Q and less than any adjacent band (if there is any). You may also apply a scaling such as $2x$ to adjust the magnitudes.

If you would like to form the analytic lowpass signal from the I and Q then you can easily form it like:

$$x_a[n] = I[n] + j Q[n]$$

where it can be shown that the analytic signal is also given by

$$ x_a[n] = A[n] e^{ j\phi(n) } $$

where $A[n] = \sqrt{ I[n]^2 + Q[n]^2 } $ and $\phi(n) = \tan^{-1}(\frac{Q[n]}{I[n]})$

Depending on the application, information in $A[n]$ (the envelope) or in $\phi(n)$ (the phase) is used.