@miikka1978 said:
Could you specify the page where it starts?
Sure, the article is "10 Steps to 3D printing your Sketchup Model in Full Color" on p. 13, and the texturing portion starts on p. 16.
In the article, I show how to manually create a texture map and apply it to an object. For more info on how to edit the texture map in Photoshop, there is a good tutorial here: Use Image Editing Software Inside SketchUp.
In my Catchup tutorial, the unwrapping and mapping is done manually. There are plugins that may make the task faster like SketchUV and Fredo's ThruPaint, but I have not used them myself.