The first thing that is needed for a new template is to merge (join) the individual cells together.  Select the cells and click the Merge button on the Design section of the ribbon. The merged area is then changed to a cell:

Pressing the Merge button again will un-merge the cell.  The shortcut key F4 will have the same effect.

The dimensions of the merged cell can be viewed in the right hand corner of the screen as the grid squares are selected:

If there is already an object definition in the cell it is good practice to reset it (click the Reset button from the ribbon) BEFORE un-merging the cell.