Cutting the pieces:
Grab a dress that fits your little lady, to use as a pattern piece. Fold the dress and your fabric in half and line up the folds. Trace to the bottom of the armhole. From here, you want to trace it out on an angle because you want to have plenty of room, so when you tie it, it will gather for ya.
Now, for your other pattern piece, you need to cut the ties. You're basically going to make a rounded off triangle.
First, cut four isosceles triangles that are 6 inches by 16 inches or so. See my tutorial on cutting shapes. Super easy.
Once you have these cut, place them together and round off the pointy tip.
For the side of your triangle that's going to hit the side seam of the dress (the short side), you want to get that angle. Place the triangles you just cut up against the side of the bodice and cut that angle.
You'll end up with this:
Here are all your pattern pieces.
Sewing this dress is pretty darn simple. First sew the ties of your dress, leaving the slanted edge (that will be on the side seam) open. Turn right side out, press and top stitch.
Pin the shoulders, right sides together. Also pin the sides of the dress, but make sure you sandwich the ties in between. Place them 1-1.5 inches below the armhole. Sew up the shoulders and sides of the dress. If you'd like to serge or zig-zag stitch to give it more finish, go for it. (But knit won't fray, so it's not necessary.)
Now since you cut a generous A-line, and you'll be tying the front, we need to cinch the back somehow. On the back of the dress, measure 2 inches down from the arm hole and mark. Do this on the other side, too. Then measure 1 inch up from the bottom of the tie and mark on both sides.
Now draw lines across the back that you'll follow to cinch some elastic. Sorry, they're tough to see.
Cut two pieces of 1/4-inch elastic to half of your recipient's waist measurement (for my daughter, that's 10 inches).
I didn't take pictures of this next one, but you want to put a mark at the middle of each piece of elastic and in the middle of the dress. You're going to zig-zag stitch the elastic on, stretching it as you go, lining up the mid-points. Then stretch the elastic to the other side while you zig-zag. It will gather in your fabric, so you'll have this:
Now to finish up the neck and armholes, you'll want to finish them off with bias tape. If you need help with that, check out this tutorial from kitschycoo. I like how she gives her rational for everything. Very helpful.
Then just hem the bottom!
I like how the neckline is a little boatneck-like... a little. :)
Try it on, give it a tie, and there you go. Fancy schmancy! And perfect for summer. Oh, summer... when will you return?