Usually these are ellipses, but not always.
The book "The Complete Guide to Perspective Drawing: From One-Point to Six-Point" by Craig Attebery is methodical and comprehensive. It provides a number of different methods for drawing ellipses in chapter 8 "Ellipses, Spheres, Spiral Forms, and Random Curves":
My preference is to use the Twelve-Point Ellipse method. However I modify it, adding one more point on each diagonal, making it a "Sixteen-Point Ellipse".
The above is Craig Attebery's method. Then, because I tend to have the most difficulty in the corners, I add this little tweak to the diagonals:
I believe the point should be drawn ~30% of the distance from the inside blue line to the outside blue line, but I usually just place it midway between the two.