Story boards are essential for designing the flow and the plot of the game. Once the storyboard has been assembled, parts will slowly be filled in. (Puzzles, achievements and other bits will be incorporated after the in game plot/backstory assembly addin.
Let's say you wish to edit the starting frame. Find the sketch of it, open it and then click 'More Info'. From that page, start editing.
If you wish to insert a new frame, click 'Add to Gallery'. Then upload your photo in the same format as the example. Make sure you put it in the right place!