In order to apply textures to a model, you have to define where each pixel on a two-dimensional map will fall on the three-dimensional model. In order to do that, you have to create a UV map, which takes the three-dimensional wireframe model and "flattens" it into two dimensions. Once you make the UV map, you can use it as a guide to paint a texture in a paint program. Usually what people do is take the UV template into Photoshop or a similar program that you can work with layers. Textures are added on layers above the template so that they get applied in the right place. Hope that helps.