This project assumes you already can sew a basic shirt.
Tunics. All the kids wear them these days! They're comfortable, fashionable, and otherwise awesome.
Dolls need more. Away with those t shirts! Let's embrace fashion! Let's dress your doll up so that she's not humiliated to go out in public and socialize with other dolls!
In addition to everything you need for shirts, you need:
Ribbon -- Actually, you can get away without this
if you want, but I'm going to strongly recommend it.
I know Simplicity makes a pattern that turns out similar to this, but not the same. The Simplicity one looks funny when it's all made up, methinks. Look, try it my way at least once before you run screaming to the craft store. I can save you a headache and 99 cents.
Cut out whatever sleeve you like (bell sleeves work nice), a bodice front and back pieces cut VERY VERY short (think right under the arm pits), and a long rectangle. The width of the rectangle should be 1.5 to 2 times your doll's waist line plus 1.5 inches, and the length should be the measurement from the doll's arm pits to wherever you want the tunic to hit, plus an inch.
Start making it up like a normal shirt. Sew those shoulders together!
In fact, if you wanted to, and you do, you could finish the thing up pretty far by putting the sleeves in, zipping up those side seams, and putting a collar in. I sewed ribbon on for an edge, but you use the collar you like.
Hem the large rectangle on one long side. Gather the other long side, then pin it to the tunic, right sides together.
Sew it up.
Now, take the ribbon and cut a long piece. Don't worry about measuring, just pull a piece as long as your arm or so. Pin the center to the front center of your tunic, right on that center seam. Now, making sure to cover the center seam, keep pinning, working your way out to the edge. I'd stop pinning at the arm pits, but you're welcome to go all the way to the back if you're feeling it.
Stitch it down ON BOTH SIDES of the ribbon. Once again, I'd stop at the arm pits (you'll see why in half a second), but you do what you like.
Finish it up in the usual way, adding velcro to the back. Hey, if you stopped at the arm pit like I told you to, you should have two ribbons flopping around, right? Check it, here's what you do with that: