These videos are a few years old but they might be useful. All of the were made before the current version of Blender. Blender Cycles is now much easier to texture paint with than it used to be. Other than that difference, these are good visualizations of how you can paint a skin on a mesh avatar. The process is easier with standard avatars (often referred to as SL avatars in these videos) which are already unwrapped vs other hand made or Make Human avatars where you need to mark seams and unwrap.