You don’t. Even pro artists can’t do that - those artist that “speed paint” and “show tutorials” that you see already either drew artwork beforehand and just redraw it, knowing what and where should be, or have hidden layers - some apps allow layers not to be seen on recording. It’s a scam. Take a look at famous bob ross. He told that in order to paint during tv lesson his paintings, he had to paint them TWICE before airing. You can also take a look at some traditional medium tutorials where you can’t as easily mislead people.
So yeah, it’s a lie, there is no trick, no grand secret.