As you can see in the video, you doodle random shapes onto the giant canvas that you can see at the start of the video, then you can take a photo of it and overlay it onto a sketch of your painting. You can then use the doodles on the big canvas as guidelines to see where something should go.

Random doodles are much better than a grid because drawing it is faster and more fun and you can find positions quicker (as you don't have to count lines constantly).

"So let me get this straight," Harry said as it seemed that Ron's explanation (with associated hand gestures) was winding down. "Catching the Snitch is worth one hundred and fifty points? "

"Yeah -"

"How many ten-point goals does one side usually score not counting the Snitch?"

"Um, maybe fifteen or twenty in professional games -"

"That's just wrong. That violates every possible rule of game design. Look, the rest of this game sounds like it might make sense, sort of, for a sport I mean, but you're basically saying that catching the Snitch overwhelms almost any ordinary point spread. The two Seekers are up there flying around looking for the Snitch and usually not interacting with anyone else, spotting the Snitch first is going to be mostly luck -"

"It's not luck!" protested Ron. "You've got to keep your eyes moving in the right pattern -"

"That's not interactive, there's no back-and-forth with the other player and how much fun is it to watch someone incredibly good at moving their eyes? And then whichever Seeker gets lucky swoops in and grabs the Snitch and makes everyone else's work moot. It's like someone took a real game and grafted on this pointless extra position so that you could be the Most Important Player without needing to really get involved or learn the rest of it. Who was the first Seeker, the King's idiot son who wanted to play Quidditch but couldn't understand the rules?" Actually, now that Harry thought about it, that seemed like a surprisingly good hypothesis. Put him on a broomstick and tell him to catch the shiny thing...

Ron's face pulled into a scowl. "If you don't like Quidditch, you don't have to make fun of it!"

"If you can't criticise, you can't optimise. I'm suggesting how to improve the game. And it's very simple. Get rid of the Snitch."

"They won't change the game just 'cause you say so!"

"I am the Boy-Who-Lived, you know. People will listen to me. And maybe if I can persuade them to change the game at Hogwarts, the innovation will spread."

A look of absolute horror was spreading over Ron's face. "But, but if you get rid of the Snitch, how will anyone know when the game ends?"

"Buy... a... clock. It would be a lot fairer than having the game sometimes end after ten minutes and sometimes not end for hours, and the schedule would be a lot more predictable for the spectators, too." Harry sighed.

• Harry Potter and the Methods of Rationality