When I saw this dress on anthropologie earlier this year, I knew I wanted it... but I also knew it was way out of my price range. And even if I could afford it, it's no longer available. SO. I decided to make one!
You'll need: a plain white cotton dress, fabric paint, brushes, water
I dampened the dress, then laid it out on my table with a board in the dress to prevent the colors from seeping onto the back. Then I just painted! I did it freehand (which is probably why it doesn't look *exactly* like the original lol) but you could always sketch out your design first lightly with pencil, or even trace it from a large scale landscape print-out.
As long as the dress stays damp, your colors will bleed a bit, giving it the watercolor look. I'd recommend ironing it really well before you try to wash it, to lock in the fabric paint and keep it from running in the wash. Give it a couple days to dry before wearing it or washing it. Mine was still a little damp after 2 days so I just let it hang out for a while.
I got my dress at Dorothy Perkins, but plain white dresses are really everywhere! As long as it's cotton it should work! :)