You'd mark along the edges of the slats and wind up with dashed lines. You can then either connect the dashes with the marker before sewing or just connect them by sewing through the spaces. I'm just guessing, of course, but it's the only way that makes sense to me. Only SweatyPie knows for sure how her guild does it.

Personally, I'll stick with my walking foot bar and skip the bother of marking/taping any more than one line in each direction.