I appliqued the outer edges of the circle onto the background by eye. As you can see, it's not exactly circular (more scallopped!) but hey, it's good for a first attempt. And here is the next one in progress.
I didn't like the plain blue outer ring, it didn't add anything, so I undid it and tried something a bit lighter.
The camera has washed it right out, but you can see better on this close-up.
Next decision will be the centre. Pink? Royal blue? Green? Any ideas?