The drawing is made directly on the skin, this way we make sure the painting will flow with the body's anatomy, the tattoo will belong to the body.
For the abstract artwork brushes and pens are used to create on the skin. For the figurative tattoos, a stencil can also be applied as long as the same process of the abstract ones.
In some cases a study is made on paper to define specific questions, never as a final art. As the skin doesn't work as paper and I tattoo people, not living canvas, the design is made to work with the body. For canvas the process is completely different.
If your goal is to have a tattoo that looks like a painting and moves with your body, the creation of the design directly on the skin is the best option.