Is the dress sleevless? What kind of skirt does the dress have? If you take in the bust at the side seam, and the skirt has sleeves, you would have to take them out and alter them slightly as well. If the skirt is fitted you might be able to get away with simply taking in the side seam. If the skirt is gathered you should take it off first and then take in the waist at the darts and side seam. And then alter the skirt to fit the new waistline.