I took a cute 2 tiered chiffon skirt with an elastic waist and pulled it up over my dummy's boobs. Then I chopped off the legs of a pair of wide legged pants for the sleeves and belt. My 15-year-old likes it so that seems like a victory to me!
Measure from your bra strap to the desired sleeve length. Add 2" and cut off the bottom of the pants to that length:
Measure from your bra strap, horizontally, to the middle of your arm pit. Add 1" and open up one of the leg side seams (starting at the cut edge, not the hem of the pants) that much:
Make an elastic casing at the raw edge of the sleeve and thread through elastic. Secure the elastic to the casing opening that will be at the back of the top. Safety pin the elastic at the front opening to be adjusted later.
Pin the sleeve opening to the top of the skirt. Sew, starting at the center of the arm pit and working out. If you've used an elastic waist skirt as I have, stretch the skirt elastic out as you sew on the sleeve. Stop before you get to the front sleeve elastic opening. Try on your top and adjust the elastic to your comfort, then secure the elastic and finish sewing the sleeve to the skirt.
Cut 6" wide strips of fabric from the remaining pants legs and piece them together to your desired length. Sew the strip with right sides together, turn and press for your belt. You can use any kind of fasteners you like to complete the belt, hooks, snaps, even a buckle.
The top looks cute without the belt, too.