Re:
Yes, you usually use a reference photo or photos.
You can do it in photoshop but I think suits Illustrator. You basically draw a load of triangles for the areas of light and shade then fill them.
In Illustrator I just use the pen tool to draw the triangles (with a stroke, no fill) then use live paint to fill the areas. You'd do it in a similar way in photoshop. There are some youtube tutorials showing how to do it in photoshop but not tried them.
There are some apps out there that supposedly convert a photo to a low poly picture but the couple I played with were pretty hopeless.