To make it I simply cut out a strip of cloth (actually it was two strips- I had side seams) for the skirt. I sewed it into a loop and then I hemmed one side and gathered the other. Then I flipped the skirt inside out and upside down over top of the onesie and pin the gathered edge on at the waist area. Then I sewed it I think using a zig zag stitch so that it would stretch better. You could stop at this point but I decided to put something around the neck too.
I cut a narrow strip of the floral cloth and then I finished one edge by sewing over it with zig zag that was set on a really short stitch. (Similar to what you would use for applique.) I let the knit fabric stretch a little as I sewed so that it would make it a bit ruffly. Then I gathered it up along the neckline and sewed it on again with zig zag. (I also finished the edges at the end of the strip.) Then I sewed a narrow ribbon over the area where I sewed the ruffle on.
That's it! It was easy and fun. I love upcycling like that. :-)