How do people who make graphetti get good at it?

Although I've never done that (nor do I know how) I've seen it on train cars and other places and lots of it looks very artistic. (Which I love art.) I can say that those who've done it are great at art (in my opinion because the appearance is very creative.) I don't know how they get it the form they want; but, (looking at the way it's done) I believe that they should teach others how to create something that is that artistic. So, how do you create it and arrange colors, so that they appear that perfectly? Really, I'd love to learn how to make it. I look at details and that really looks good.