How to plot the Hilbert Spectrum in Hilbert-Huang transform?
Final step is pretty straightforward. All you need to do is to apply the Hilbert Transform to each IMF and extract the instantaneous frequency from analytical signal. Instantaneous frequency is given by:
$$\omega(t)=\dfrac{d\phi(t)}{dt} $$
where $\phi(t)=\mathrm{arg}[x_a(t)]$ (unwrapped phase of the analytical signal). Keep in mind that MATLAB (Octave) hilbert function already returns analytical signal.

In fact you might want to re-use this code from Mathworks File Exchange: plot_hht it is a good starting point.