Question on sketching the amplitude spectrum of signals
It's really very simple, but should have been clarified in your tutorial. It may turn out that a signal's Fourier Transform is real (I'll let you figure out when that happens). In that case, the spectrum $X(f)$ can be drawn in a single plot using positive and negative numbers, without having to split it up into magnitude and phase spectrums.

You can, of course, represent a real number in polar form. If the number is positive, its phase is zero, and if it is negative, its phase is $\pi$. What this means is that, whether you plot the real Fourier transform using negative and positive numbers, or you split it into magnitude and phase plots, you're conveying the exact same information.