Start with a given ellipse with its major and minor axes shown. 

1. With the compasses' point on the center, set the compasses' width to half the width (major axis) of the ellipse. 

2. Move the compasses' point to one end of the minor axis of the ellipse and draw two arcs across the major axis. 

3. Where these arcs cross the major axis are the foci of the ellipse. Label them F1, F2. 

4. Done. The two points F1, F2, define the foci of the ellipse. 
