I drew a still life of my phone, some scissors and an eraser without any shading. I then scanned it into photoshop and chose the right contrast of black and white.

I then used the magic tool to select areas that i wanted to colour so it would not go over the lines. The opacity of the paintbrush was 10% so i could layer the different shades of paint.

After I coloured the objects I changed the opacity to 5% and used black to layer on the shading around the objects.