Measurements
Put the dress on inside-out and pull the side seams away from your body until the dress feels fitted. Secure the extra material at the side seams with straight pins. Sectioning off the same amount of material on both sides ensures the back zipper and any front designs are in the appropriate location after re-sewing the side seams. Carefully unzip the dress and step out of it to avoid disturbing the location of the straight pins.
Stitches
Keep the dress inside-out while you sew so the seam lines are hidden from the outside. Use a sewing machine or sewing awl to create a lock-stitch. A lock-stitch is an extremely sturdy sewing technique that locks the thread together each time the thread crosses the fabric. Another benefit of using a lock-stitch is if one thread section breaks, the seam will not unravel because the thread is secured to the fabric each time it crosses another section of thread.
Cuts
Cut off the old seam line after a new seam is created and secured at each end. Cutting off the old seam directly along the old seam line and leaving a fair amount of extra material allows you to let the dress out at a later time if you want more room in the chest and stomach. Hem down the newly cut edges to prevent the fabric from unraveling. Lock-stitches can be placed close together, although no matter the closeness, they may not prevent unraveling fabric from making its way past the seam line.
Finishes
Separate the extra fabric at the seam lines by pressing down the middle and running a warm iron to create a crease. This process holds the extra fabric flaps along the inner bodice so the seam lines do not appear bulgy from an outside perspective. If the material is not iron-friendly, lay a towel over the garment̵7;s open seam and press a warm iron onto the towel so the warmth is transferred but the hot iron plate does not disturb the material.