I did this back in June. It was simple though the hand stitching was a little time consuming. Definitely a project you could do in an hour (if you didn't have babies climbing up your leg or running around the house tearing books off your bookshelf.)
I cut the flower out of pink felt (and a matching piece of Heat N Bond) using my Cuttlebug and a flower Nestabilities die. I ironed the Heat N Bond onto the back of the flower and then the flower onto the onsie. I used embroidery floss to secure the button and to do the hand stitching. And BAM! You have just prettied up a plain boring onsie.