You found a vintage dress in a shop that is nearly your size, but not quite. Or perhaps you have a favourite dress bought years ago, when you were a smaller size, and don't want to throw it away. You can still buy that vintage dress or wear your old favourite by letting them out. However, most commercially-made dresses from the past few decades cannot easily be let out because there is insufficient fabric in the seams. But you can remedy this by adding extra fabric to many dresses.
Determine whether the dress can be let out. Examine the seam allowances (the fabric on the inner side of a seam) to see if there is enough fabric to let out the seams and still have some fabric left for a new seam allowance. If not, letting out the dress would involve sewing in panels of matching or coordinating fabric, which may or may not be desirable.
Try on the dress and determine where the dress is too tight, and by how much. Open the seams in the areas where the dress is too tight be carefully clipping the seams. Try the dress on again, and pin the seams back together so that the dress is comfortable. The new seam allowances will be pinned to the outside at this stage. If there is insufficient fabric to allow for new seams, cut a panel of matching fabric and pin it to the dress, adjusting to the shape of the dress and the seams, and cutting away any excess fabric from the panel. You can also use coordinating or contrasting fabric for interesting effects.
Mark the new seams with tailor's chalk, and pin the dress again, this time with the new seam allowances on the inside of the dress.
Sew the new seams. Press the seams.
Don't use a seam ripper to let out the seams--you'll have more control and be less likely to make holes in the fabric of the dress if you use small scissors.