How hard is it to learn to set a mosaic portrait from scratch? Not as hard as you might think, provided you learn in such a way that you understand each stage.
If you look at the image here you can see how the 2 black lines create the basic form. Dependent on whether the face is full on, side profile, 2 thirds/3 quarters profile will determine where the limits of the face extend out from these 2 lines.
Now look at where the main features are, the eyes, nose and mouth sit on the lines, the line of the jaw extends out in relation to the profile (as above).
That's it, you have the start of your portrait. You can learn how to set eyes etc from looking at other examples and picking the style you like best. Same for the hair and costume.
Look at an anatomy drawing to know where the shadows will be according to bone structure and then is your use of setting the tesserae and choosing your colour shades.
Always work in stages, 2 lines - eyes, nose, mouth, line of the jaw - dark areas - lighter areas - other shades in between - hair - costume. This way you get each stage correct before you move onto the next, that way if you ever get stuck there is always a point you can go back to which you are sure of. Make sure that previous stage is correct then move on again.