A skin is typically just a texture, though in a more complicated paradigm it could be extended with other types of material info, custom shaders, effects, etc... But at it's base, skinning is just changing the texture that is mapped onto a model.

How you do this is pretty dependent on what application you are using, how the model is handled and rendered, what APIs, if any, you are using, etc... In short, you need to provide more info about what you want, since that is one hell of an open-ended question you asked there. [grin]