Trial and error.
Sorry I don't have pics to post, but I was a little too frustrated to think about it at the time. The dress itself is a non-fitted design, and I believe I could have done the searing without increasing the width, or with minimal increasing. Needless to say, it was too big. And then I gave too much at the top for the elastic and made the area under the arms too small. It took a little adjusting, some seam ripping, but I got it to work.
I took some of the contrasting fabric and made some straps (albeit too long-I'll fix that later) and sewed them in place on the back of the dress.
All in all, it turned out pretty cute.
Now it's time to start planning Bubba's 4th birthday party...Disney's Cars.
What do you think of the party and birthday dress? What party plans are you making? I'd love to hear from you!