Anneadeline, isn't it, though? I'm thinking of making one in every color.
OK, simple tutorial, of sorts. Just ask if something doesn't make sense, I have a bit of a headcold.
First, I found a simple T-shirt and a basic skirt I like, to get some general measurements and shapes to start from. I laid out the T-shirt on some taped-together newspapers, and drew out the basic shape. Then I did a lot of measuring across the shoulders, bust, and waist - this fabric was 100% cotton and doesn't have any stretch to speak of, unlike the T-shirt in question - and adjusted accordingly. I made the bottom of the bodice end just at my "true" waist, which is the narrowest part of the torso, so that the skirt could flare freely. (Obviously this means I measured the length from shoulder to waist, too.)
(All the newspaper pattern pieces were measured and cut folded in half, so that they were symmetrical. Makes it easier. I folded the fabric piece in half to cut out the neck, so that it would be too. The front neckline goes down further than the back. I'd probably recommend doing a V neckline if you're not very experienced, as this doesn't have as smooth a hem as I'd like.)
I then measured the bottom of the skirt, and added on several inches to give it some oomph. This measurement, and the waist measurement, I divided by eight. (I also measured the distance between my waist and my knees, which was the length I wanted.) I made a piece that was waist/8 at the top and bottom/8 at the bottom, and cut out eight of those. (Again, fold it in half, use half the measurement, draw a straight line from each outer edge, and cut, and it'll be symmetrical.)
I sewed the top together at shoulders and one side and hemmed the neck and sleeve holes. (I leave allowance and hem after sewing together, which is tricky sometimes - if you're surer on your hemming, you can do that first.) I did two tucks across the chest, but that depends on your shape, size, and material, so you can figure that part out for yourself. Or google.
I also adjusted the side seam a bit after fitting - I did a lot of running back and forth to the mirror and pinning, which was annoying and time consuming but meant I didn't have to take the dress apart and fix stuff afterwards... I really want a dummy now, though.
I sewed together the eight pieces of the skirt, but left part of the last seam open at the top - I'd measured the zipper - so there would be room in the side of the bodice plus the top of the skirt. I put the bodice upside down inside the skirt, right side to right side, and sewed them together, again leaving the one side open. I then put in a zipper, which has its own tutorials.
Finally I hemmed the bottom.