Get a nice curve shape by using interfacing, sew it on the right side of fabric, cut a slit in the interfacing and turn it right side out. I always pink my circle edges at 1/4 inch seam allowance before turning so I get a smooth shape. I have also used a used dryer sheet in place of interfacing.
After the circle is turned and the edges are pushed out with a curved instrument ( I use a wooden spoon) I press with the iron. I like to use a machine buttonhole stitch to stitch it to the background. I starch my background fabric so it has more body or you can sew it with a stabilizer behind the background fabric to prevent puckers. I have used a paper coffee filter for a stabilizer in a pinch and carefully tear it away after.