I wrote POP by myself because I learned to draw, and I think it's quite simple. Now there are some kinds of copybooks on the market. You can buy several different kinds of copybooks at home and practice it well. It's not difficult to master the brushwork. Don't go to those crash courses, which are deceptive. You can't learn anything at all. The key depends on your understanding. I wish you a good painting as soon as possible! It must be good to practice and copy more! !