When I put ruffles on stuff for little babies I like non-fray chenille. It is that soft stuff that people make petticoats out of. I like it because it is soft, comfy, and you just have to cut it and sew it on.
One word of advice when it comes to putting ruffles on the butt of things, make sure you keep the ruffles low enough down or else you get a ruffle back onesie.
These are some onesies I made for my aunt when she had twins. For the words, I used the freezer paper stencil method. This was before I had a silhouette, so I had to cut them out with an exacto knife. I think being awesome at freezer paper stencils is an acquired skill. For the ruffles, I cut a 10 inch length of the non-fray chenille, cut that into 3 pieces, gathered them down the middle, and sewed them on.
I recently had the idea to take ruffle butts to the next level. I give you the Bustled Butt onesie. :)
To make the flowers, just cut circles out of any synthetic fabric. Then hold the edge of each circle over the top of a flame. I know people who like to use lighters for this, but I like to use a candle. You don't have to stick the fabric in the flame, just close to it. I made about 6 little flowers.
To create the bustle cut your chenille strips about 2.5 inches by 10 inches. Then gather them at the top and sew them down.
On the front I used a little pearl, but on the back I didn't want a Princess and The Pea situation so I just sewed the flowers on at the top of the bustle.