I find the fun of it is trying to do it fast and stealthy at night. About asking people, it's a good way to get a nice piece in a prominent place, but it's not graffiti, it's just a mural.
And about learning it, just try writing a word large on a piece of paper. Then with a pencil, start to thicken and change the letters. Add new lines, add new shapes, twist old ones. Eventually, you just hit on something that's comfortable and easy for you, and of course looks good.