My daughter has been living this these "princess" dresses lately. I have made so many lately and have been asked how to make them - so I thought I would share.
T-shirt or onesie
1/2 - 1 yard of fabric for the skirt
First decide where you would like the skirt to start (natural waist or empire waist). Then cut off the appropriate amount off the bottom of the t-shirt. You can also do this with a onesie - just don't cut off the bottom.
Next, decide how long you would like the skirt. Also, depending on your child, how full you would like the skirt. My daughter insists on having a very spiny skirt.
For the case of the very spiny skirt cut 2 lengths of the fabric. Sew a seam on one side. Then make a tight ruffle from the fabric making it the same length as the bottom of the shirt.
Otherwise you can cut 1 length of the fabric. Gather or ruffle the fabric to be the length as the bottom of the shirt.
Next with right sides together and the bottom of the skirt toward the top of the shirt, pin the skirt to the shirt. Now, sew a seam connecting the skirt to the shirt. I usually use a stretch stitch. Then I go back and overlock to prevent fraying.
Finish the bottom hem of the dress and then you are done.
You can add an applique or embellish the top of the dress if you so desire. I would love to see photos if you make a dress.