Start with a basic t-shirt or tank.
Ignore the onsie, I haven't used that one yet. Cut off the bottom of your shirt a little below the arm pits.
Cut a rectangle of fabric, I used knit fabric, that is twice the width of the bottom of your shirt.
You can have as many tiers as you want and they can be whatever size you want. I sewed up the skirt portion first and then attached it to the top. Attach each tier to each other by gathering the top to match the width of the piece before it and sew.
This little shirt already had a ruffle at the top so I added the blue and the red to finish it off. This will be a cute Fourth of July dress.
The other one I did isn't gathered tiers, but short circle skirts sewn on top of each other. I sewed a bit of ruffled ribbon on the waist.
I did a 0-3 month size using the bottom of one of the shirts that I cut. I attached this one using elastic thread so it pulled in the waist a bit. I sewed on more ruffled ribbon around the hem.
And of course, no little lady can have new dress without a decent hair bow to match!
This method could easily work for an adult size. I've been dying to try it, but with only seven more weeks of the baby bump to go, I don't know that I'll try it just yet. Then again, seven more weeks is a loooong time so we'll see. =)