A strapless formal dress must fit properly at the waist in order to look good. If the wearer must constantly tug at the top of the dress, it most likely needs to be taken in at the waist. The goal is a dress that stays in place at the waist and does not slip down when the wearer moves her body. This alteration is usually done at the side seams, assuming the dress has a center back zipper.
Fitting the dress to the wearer
The person wearing the dress should try on the dress while wearing exactly the undergarments she plans to wear, including strapless bra, slip, and/or shaping undergarments. Pin the amount to be taken in at the side seams, placing pins parallel to the seam line and being sure to take in equal amounts at both sides. Taper the pinned line to join with the existing seam in the skirt and near the top edge. If the waist is pinned correctly, the dress will stay in place without the wearer needing to pull up on the top edge. If the dress slips, move pins to take in a little more at the waist until the dress stays in place. Help the wearer take off the dress, being careful of the pins.
Sewing the new side seams
Mark the new seam lines on the wrong side of fabric with tailor’s chalk or dressmaker pencil and remove pins. Baste side seams together, keeping lining separate from outer fabric. Remove any boning, sequins, pearls, or beads that lie on or within the new seam allowance. Stitch along new seam lines, sewing only the outer layer of fabric if dress is lined. A zipper foot is helpful if you must sew near beads or pearls. Next, sew the new lining seams if dress is lined. Press seams to one side. Replace boning if necessary.
Final fitting of the dress
The wearer should try on the dress again to make sure alterations are accurate. Once a strapless dress fits correctly and does not slip down, the hem may be marked and alterations completed with confidence.
Variation for side zipper
If the dress has a side zipper and needs only a small amount (less than an inch) taken in at the waist, the center back seam may be pinned instead of side seams. This gives the seamstress only one seam to alter and avoids resetting the zipper in the side seam.