I want to cell shade my figures. I have seen many tutorials but not for houdini.
How could I make it in houdini?

I think there's an included toon shader (OGL2) that may do what you need.

chronos
I think there's an included toon shader (OGL2) that may do what you need.

where?

if you render with the Wren output driver it will create an ink outline. there is a standard SHOP type for a basic toon colour shader. composite the Wren pass over the colour using COPS.