If you've made a couple of practice leotards from the Kwik-Sew Swimwear and Activewear book, you'll start to get pretty comfortable working with sleeves. The biggest problem with sleeves is getting them to lay flat when you raise your arms into dance position. Two things, the armscrye and the sleeve cap height, are what make the difference here.

The armscrye is the size of the opening where your arm goes. In a ballgown you want the armscrye to be snug around your armpit/shoulder. I'm assuming once again that you are working with stretchy fabrics. I usually determine if the armscrye is alright by making a test bodice first, and then trying it on. You can also estimate the finished armscrye by measuring the pattern pieces and then subtracting the seam allowances. The distance around should be the same as your arm circumference at the armpit, or even a little smaller depending on how stretchy the material is. If you need to make the armscrye smaller, the easiest way to do it is to take the excess out of where the shoulder seam would be.

Once you've gotten that worked out, you will need to adjust the height of the sleeve cap. The sleeve cap is the bell-shaped part at the top of the sleeve. The higher (more belled) the sleeve cap, the more room it will give you in the shoulder, and so the more poofed the sleeve will get when you raise your arms. You'll want to flatten out the sleeve cap in order to eliminate the poofiness.

What I usually do is first determine the finished armscrye measurement. I often sew the bodice and measure the actual assembled garment, rather than relying on measuring the pattern. Whatever the measurement is, the sleeve cap from arm seam to arm seam -- i.e. along the edge of the bell -- needs to be the same (and don't forget to account for the seam allowances). So, if the armscrye is 14 inches and the sleeve cap is 16, that means that I somehow need to take two inches out of the sleeve cap. You do this by lowering the height of the bell.

I don't have an exact science for flattening the sleeve cap to match the armscrye, but this is what I do. I start by marking the center of the top edge of the bell. Then I mark down an inch from this edge. I use my dressmaker's curve to re-draw the bell, using the mark I just made as the new top center of the sleeve cap, and fitting the curve to use the same outer edges of the bell as before. Next I measure this new curve to get a sense of how close I'm getting to the desired size. If I'm within a 1/4 inch I leave well enough alone, and cut along the new curve and construct the sleeve from there. If not, I try again, a little lower or higher as needed.