There are several ways can be used to create a truncated cone with Google SketchUp.
The first way is to use scale tool combined with the use of Ctrl (Control) key on the keyboard. Ctrl key is useful to center the top of the cone still in the middle.
Selection the top surface. Use scale tool, and use it on one corner. For more clearly, please follow this interactive tutorial below. Please note, because it is an interactive tutorial so you need to click the Play/Next button (and following button) manually.
Related Google SkecthUp Tutorial: