As the others have said, carefully take the dress apart. I would make some notes and take pictures before you start and as you go along. When the pieces are taken apart, you can lightly starch them to help them hold their shape to make tracing a bit easier. And, yes, remember to add seam allowances and make sure you whatever seam allowance you decide on that it is the same on all the pieces. You should generally use somewhere between 3/8 and 5/8 seam allowance for clothing. Good luck!