Like the others have said, you need to do an apprenticeship. It's the best way to learn. You're learning from someone with experience in the field and you can watch them work. They will often ask you to put together a list of items for a portfolio. I know for one I had to do a lot of traditional designs like 2 versions of a tiger, a death before dishonor tattoo, and more. Any more ?'s feel free to
PM me.
Some tattoo artists aren't great on paper but are amazing on skin. It's a matter of understanding how the machine works, how to use it, colors, same with the needles, and sizes of the pack and what not.