Start by hold your phone like you were reading a book, or maybe a bit higher yet.
Then rotate it into an orientation as if it was lying flat on a table.
Continue that rotation further, until you reach the reading position again.
If everything works as intended, you should see a camera preview and a button "sketch."
Normally, you do not need the button: Holding the phone upright and still should already trigger it.
But if that does not happen, you can start the sketch using that button.