the donut foot is used for free motion quilting
they usually have a spring so the foot can bounce on your quilt.
you will need to lower your feed dogs with these
and adjust your stitch length to 0
the stitch length in FMQ is created as you move the fabric
tension will depend on your fabric and thread
just play for awhile- change the settings and see how they make a difference.
this will help you get the look you want on your quilt.