I got a whole bunch of lovely vintage fabric from Etsy for pennies so I have been pootling about making things all week and this is one of those things. This the first proper tutorial with pictures that I have done, so any input from you would be marvelous, as would any pictures you might like to send me of the fruits of your labour. This design is best suited to fabrics that don't unravel too easily. Also, be aware that it is unlined, so don't go for something too transparent.
So you need to start of with a trapezium of sorts. The straight vertical edge which you will place along the fold is 18". The top edge is 6" and the bottom edge is 9", and you join the ends of these two lines to get your diagonal. Round off the lower edge as shown in the very complex and state-of-the-art diagram below. For the front piece, you then measure 2" along each side from the top corner, and cut a curve (represented by the dotted line) to make your armholes. NOTE: I have improved upon the original ever so slightly and decided that, for best results, rather than cutting two pieces identically as shown below, the back piece needs a slightly narrower back, so for one of your pieces you should measure 3" along the top and 2" down the side to make a slightly wider curve.
This particular dress is designed for my nearly 3-year-old, but for moderation purposes, I would suggest increasing the length by 2" and the overall width by 1" for each size increase, so for a four year old your trapezium would be 20" high, 6.5" wide at the top, and 9.5"wide at the bottom. I haven't road-tested this theory though, so you may have to play around. Try measuring some existing clothes and decide what you think will work.
You need to cut two of these pieces, as well as one rectangle measuring 7" x 4", and two long strips measuring 24" x 1.5".
I like to get the fiddly bits out of the way first, so I started by pressing the long strips, which will form the straps of the dress. Fold in half lengthways, and press....
... then fold the two raw edges under to the centre and press again. Repeat with the other strap, and that's the really irritating fiddly done with.
Fold the 7" x 4" oblong in half and press, then tuck the raw edges under and press those too.
Using a long stitch and a low tension, sew across the top of your front piece, from the top of one armhole to the other and pull the bottom threads to gather it in until it is the same width as the rectangle you just pressed, securing the ends when you have it right.
Pin the gathered edge between the two folded edges of the rectangular piece, and sew in place. (don't forget to readjust your stitch length and tension. I always forget.)
Hem the top of your back piece, then join the sides of the dress, finishing the seams however you prefer, and hem the bottom. No pictures for that bit, but I'm assuming you know what it should look like.
Find the middle of one of your straps, and align it with the side seam at one of your armholes. Sandwiching the raw edge of your armhole between the two layers of the strap, pin carefully all the way around the armhole.
Starting at one end of the strap, sew along it entire length....
... around the armhole and all the way to the other end of the strap.
Finish the ends of the straps, tie them up on your model's shoulders (you can put in a few hand stitches to secure them in place if you prefer, or just leave them loose so that they can be adjusted as you like) and you're done.