The first thing to do is to draw a circle with an arc along the edge. For this example, I set my circle's radius at 5'.
When drawing the arc, I zoomed in until I could make out the endpoints of the polygon used to render the circle. (In my SketchUp settings, circles are actually 24-sided polygons. I know it is possible to adjust this setting to increase or decrease the number of sides) I drew the base of the arc between two endpoints on one of the circle's "sides" and pulled the bulge of the arc in towards the center of the circle about 6".