You will have to follow a certain direction to keep the skin the style and design we have but you will also be free to explore and design new skin. This risk to be a difficult process since the skin system is far from being complete at 100% so don't be surprised if in the next month there is major changement in the way we draw our skin.
You will require some program like photoshop or illustrator. You can event use some program like gimp but it risk to make the job a lot tedious.
Creating new skin
Still undetermined but something about skin
Photoshop or illustrator formation
Tips and conseil about making skin