you can make the texture with photos from real skin. or you can paint them. if you want to paint it you need special brushes for skin. for example something like this
mediadesigner.digitalmedianet.com/articles/viewarticle.jsp

you need also a good knowledge how to work with UV maps. i dont know for any good tutorial. i could now go on google and search for them but you can do this yourself. type in google : 
-photoshop skin texture tutorial
-photoshop painting skin
-3d skin photoshop

something similar.