Creating window sills in SketchUp is part of the process of creating components. We go through the whole procedure of modeling components for architecture and interior design in our SketchUp training courses.
After a component has been initially created it may require further editing, for example to a simple window component you would possibly add details later on like sills, mullions, etc. To do this first select then right-click any of the windows. Note the three main options which appear at this point: Edit Component, Make Unique and Explode. In this case choose Edit Component which will update all instances of the window.
Next create two horizontal cut lines at the sides of the bottom panel and use the Push/Pull tool (hit P on your keyboard) to push the panel outwards by 20mm, thus creating the window sill. The extrusion value can be seen at the bottom right of the interface as you type the number in. Then choose the Select tool and select the bottom line. Select the Move tool and move outwards by 20mm giving you an angled sill. Then create an arc with the Arc tool on the top edge and Push/Pull this section across and away. You could also create a chamfer on the bottom edge with Line tool and Push/Pull this section away also. Lastly choose the Select tool and click outside the window to exit the Edit mode.
Zoom out of the scene and note that all the instances of this component have been updated, but in the case where you only wanted to change one instance of the component you would choose the Make Unique option.
To create a beveled edge on the window frames create a profile in the centre of one panel, then select the lines of the path (in this case the inner rectangle), click on the Follow Me tool and select the profile shape. In this way many and varied modifications can be made to any component – the level of detail is completely up to the user and purpose of the model.
Many useful tips and techniques can be found at the SketchUp website. And see many examples of our own clients’ work on our Facebook page.
Other related SketchUp Blog Posts:

• SketchUp background
• SketchUp introduction
• SketchUp modeling
• SketchUp contours
• SketchUp workspace