Well, a tatoo scratcher, basically, is someone who taught him/herself the art of tattooing without the benefit of an apprenticeship under a professional artist. You could apply just the term "scratcher" to anything you can learn by yourself without a formal training.
In Spanish, the person who teaches himself/herself how to do things or to speak a foreign language, for example, is called "autodidacta". It means a person who taught himself/herself.
Hope this helps!!
Native Spanish speaker and Spanish tutor