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).