Although I love crafting, my sewing skills are preschoolian, so I decided there's no reason a princess gown has to be one piece and that I will create an 'almost no sew princess homemade costume'. My princess wore a shiny white blouse she already owned, along with a "gowny" skirt I assembled (you'd hardly call it sewing) and some matching ruffles and sash.
To make a princess-worthy gown skirt, I began with an old curtain salvaged from my great-great-aunt's house (you can ask around on Freecycle for a curtain, tablecloth, or fancy sheet). Besides for being plenty cheaper, this was a lot more convenient than using new material: the curtain was large enough that I hardly had to do any sewing; and since it came pre-hemmed (and with pretty trim to boot), I didn't need to hem the skirt.
I measured my daughter's waist and cut a rectangle five times that width, on the advice of my seamstress mom. The length of the rectangle, obviously, would be the desired length of the skirt (not all the way to the floor, or your princess will be tripping all day).
I cut buttonhole-like slits all along the top edge, then threaded through a length of elastic, to gather the waist. I knotted the elastic and hand-sewed down the open end of the skirt - the one seam required.
To make a lining for my lace skirt, I cut a rectangle of plain white fabric, only 2-3 times the waist width this time, and attached it to the elastic.
I cut circles of the lace for ruffles at the neckline and wrists, and a wide sash to cover the unfinished waist of the skirt. You could use tulle for this if preferred.
There's a store-bought crown in the picture, but here's a crown I made for another daughter when the store-bought crown was too tight:
I started with a stretchy headband that fit well, a length of 6-ply copper wire (saved from some electrical work we had done last year), and some elegant plastic beads. I only used 2-ply of the wire.
The picture should give you the idea. Basically, I twisted the wire into a loop around one bead, then caught another bead in the next twist; that made one peak of the crown. I made a four-peak crown, then wound the wire 'tails' around the headband a couple of times. I cut off the excess wire with scissors. Voila - a truly elegant princess crown!