It is very difficult to stitch irregular pieces by seaming in the normal manner, ie if your fabric is an outward curve you have to bend it and stitch it on an inward curve so it will lie flat when you open it out.
One way is to draw a line the exact shape of your piece and then shape the piece to fit along the line. It would prob be easier to turn under a small seam allowance (if you're worried bout raw edges) and top stitch the piece down or use a decorative stitch as Moonpi suggested.
Water soluble glue stick is great as a temporary hold for hloding down a piece untill you've stitched it.