A well-fitting prom dress is critical for both comfort and looks. Professional dress alterations can be expensive, but you can make many simple alterations at home with only a needle and thread. If you are skilled with a sewing machine, you may even be able to re-size a prom dress, change the neckline or combine two dresses into one.
Try on the dress with the appropriate bra and shoes. Determine any fitting issues. Is the length right? Does the bust fit? Strapless dresses should fit close from bust to waist, while a dress with straps is designed for a looser fit.
Pin your prom dress to correct the fit. Ideally, the dress should be inside out when you pin it. To alter the bust, pin at the bust darts, rather than the side seams. Even out substantial alterations between seams, when possible.
Baste alterations into place. Use a needle and matching thread, or a sewing machine set to a long stitch length. For best results, sew hems by hand. If you remove fabric, stitch over the cut edges with a serger or zig zag stitch to prevent the fabric from fraying.
Try on your prom dress and assess the alterations. Make any necessary adjustments.
Tips
Practice makes perfect; try altering an old dress before you tackle the new one.