Nearly every tattoo artist will say that the first thing that someone who wants to learn tattooing has to do is practice drawing. And practice drawing anything and everything. And not just outlines, but shading and different styles of using color. Another important part of learning to tattoo is observing. Seeing how the machines are properly set up and maintained. Seeing how designs are applied. See how the artist deals with all the different people who can walk through the door of a tattoo studio.
And then there's actually learning to tattoo. Sadly some artists wind up deciding they can teach themselves and they often jump right to the unfortunate skin of trusting friends. In many tattoo traditions, the first tattoo that an artist does on genuine skin is done on their own, just so they know what their own technique feels like and so that the very first permanent piece of art happens to themselves. Prior to that, practice tattooing takes place. But what that practices happens on can vary widely.
Japanese artists used to traditionally practice on daikon radishes. Artists from India have told stories of using white onions. In her book SKIN DEEP, tattooist and author Karol Griffin describes how she was given ripe grapefruit to practice on. One she was told to cover entirely with every letter of the alphabet, while a second one was to be colored until the entire surface was shaded solidly black. Some tattoo artists have talked about buying large turkeys or hams and tattooing the surface of those.
Modern chemistry has produced rubber-like sheets of practice skin for 21st century aspiring tattoo artists. These come in various sizes and are often strapped to the surface of mannikins to simulate various areas of the body. One thing to remember as you practice tattooing: never ever will you wind up tattooing something perfectly flat when you work on human beings.