4 Marks on the cell
With the playhead at the start of the Timeline I take a standard brush and make my first mark. Using the right cursor key I then advance to the next frame and make a new mark further along the desired path. For the Video Timeline shortcut keys to be active you need to turn them on by selecting Enable Timeline Shortcut Keys from the menu in the Timeline window. I’m effectively working in a similar way to traditional cell animation: one frame at a time.