From TDN

Textures

The textures for your game should be of 256*256 in the .PNG format - This is so that you can use the textures for your terrain and for your models. The texture should consist of a flat color, Photoshop has the ability to create variants of one colour, therefore, you can create a lot of different shades. This should be one of the simplest things to do. Once you have all the textures you will be able to use all the variants as a great pallete from which to detail your world.

A flat-colored image pack is provided - Cel Shaded Pack: 121 flat shaded .PNG files - 119 flat shaded bordered .PNG files. All in 256*256 for terrain and models.

Models

Cel Shading Code
The Cel Shading code that you have or will add to TGE will create an outline of your model/terrain/interior. This process is extremetly GPU intensive.

Faking it
You can bypass the above method by setting the $Pref::renderOutline = "true"; to false; then you will need to create a second model (typically duplicating the orginial model), flip the normals and add the Push modifier and make it expand over the model. This isn't a very efficient method of creating your assets, but it will certainly do the trick. If this is for a main character, you need to assign the skin modifier to both models at the same time.