There are several reasons why you may need to alter a bridesmaid dress. Perhaps the dress is too long or too short. Often, bridesmaid dresses are purchased well before the date of the wedding. During the time between the purchase and the wedding, the bridesmaid could have either gained or lost weight, meaning that her dress will no longer fit her. Instead of purchasing a new dress, you can simply alter the one you already have.
Have the bridesmaid model her dress for you. Use the tape measure to measure how much fabric should be altered, and mark the dress according to the measurements using a fabric marker. Pin up any pieces of fabric to mark alterations as well.
Help your bridesmaid to carefully remove her dress so that she is not poked with any pins you used to mark alterations. Lay the dress out on an even surface and use the seam ripper to remove the seams where the alterations are to be made. Seam rippers will allow you to cut the thread of the seams without damaging the fabric of the dress.
Use your markings to determine where to re-sew the seam. Begin by ironing the crease of the seam flat.
Alter the length of fabric needed for the new seam according to your measurements by pinning the dress where the new seam will be. Use the tape measure to perform any appropriate measurements and to ensure that the pinned seam is straight and even.
Sew the new seam using a needle and thread or a sewing machine. Remove the pins and iron the seam when you are finished. Finish the seam by stitching along the length of the seam on the inside of the dress. Trim the edges with pinking sheers, which create a zigzag pattern that reduces fraying and prevents raveling. You may use a serger machine to add a professional finish if you wish.
Perform your alterations on a dress form to make the process easier.