I think pretty much every person into game creation or 3D modelling knows how to do this but for the select few who are new, or just havn't seen it before, here's a quick and simple tutorial...
Remember, this technique double the polygon count in your model!
1. Start by making a basic shape, ie. a cube. Colour it a block colour, such as entirely red.
2. With all faces selected, duplicate the cube then go to tools>fatboy and increase the size of the duplicate. This duplicate will be the actual outline of the shape. the thickness of the outline depends on the size of the first model.
3. Go to Faces>reverse vertex order. The duplicate should now look all black. Right click in the Isometric view, then turn off draw back-faces.
4. Make sure the larger of the two models is selected, then colour that entirely black. You should now have a cel-shaded cube.
I think this is correct. I'm going off what I remember from memory. Any problems let me know.