when you do changes in skin settings screen it applied automatically.

to apply skin on the model, select the part of the model (or whole model) that you want to apply skin, and in texture manager press set skin.

If texture doesnt appear or your model turns into a single color or some gets set of stripes, then your model is not unwrapped(not uv mapped), and this is a whole big thing itself on modelling process

modelling-unwrapping-creating textures-rigging-animating.