When you create a layout, you are automatically taken to the layout editing mode. Alternately, you can click the 
button and select Edit layout in the context menu of the layouts ribbon.
Note
To use layout editing mode, you must have required permissions.
In layout editing mode, space is divided by a grid of equal-sized squares for holding viewing tiles (1).
On the edge of the layout there are grid square fragments (2), which are parts of ordinary empty cells and allow adding new cells to the layout (see Adding new cells to a layout).