I found this cell shading tutorial ages ago on Deviant, but I've only recently tried my hand at replicating it on a new coloured piece of art. It's easier than I thought it would be although you do need a good idea of the direction and placement of your light sources to 'shine on' your characters in your art pic.

Anyway the tutorial is here: caydett.deviantart.com/art/Cel-Shading-Tutorial-270935090

And is surprisingly detailed and easy to do.

It does take some time to shade stuff though and this is mainly for digital art although it could be subbed in for traditional methods too.

I coloured this using Caydett's tute: SessKag - Happy Holidays

You also really need to use layers for this technique too. Otherwise the double shading won't work. You don't actually need clipping masks if you separate the shading layers first and then overlap them. You do need layer effects like multiply and burn on those layers too. It depends on what you're comfortable using.

~ Pyre